feat(#2099): drive GitHub Copilot through the EoS descriptor + multi-event hook bus (ADR-1239)

Fold Copilot's residual runtime-literal branches onto descriptor-driven
hostBehaviors. Several issue premises were inaccurate (verified via research)
and deliberately NOT followed: writesSharedSettings/"legacy exclusion list"
(per-runtime descriptor data, copilot's false is correct); RUNTIME_CONTENT_DISPATCH.copilot
+ installSurface==='copilot-instructions' (already descriptor-driven);
extendedHookEvents (closed Claude/Gemini enum — hooks extended in code instead);
reapply ternary (already folded, kimi #2095).

Real folds (all byte-parity — golden byte-identical for every runtime):
- src/install-engine.cts + src/surface.cts: the two `.agent.md` filename cutovers
  (_copyStaged + _syncGsdDir) unified onto hostBehaviors.agentFileExtension via a
  new exported agentFileExtensionFor() accessor (kills the two-mechanism divergence).
- src/runtime-artifact-conversion.cts: applyAgentPathRewrites' copilot skip →
  hostBehaviors.noPathRewrite:true (antigravity #2096 precedent).
- bin/install.js uninstall: the two isCopilot cleanup branches → installSurface===
  'copilot-instructions' gate (symmetric with install-time).
- bin/install.js: `!isCopilot` in the two skipSharedHooksInstall checks →
  hostBehaviors.skipSharedHooksInstall:true (copilot has no shared gsd-*.js hooks).
- bin/install.js: three dead legacy inline-agent-loop isCopilot refs removed
  (copilot ∈ _DESCRIPTOR_AGENTS_RUNTIMES → unreachable; byte-parity proven by clean
  golden + real reachable-runtime install diffs). isCopilot dropped from 4 destructures.
Zero live `runtime==='copilot'`/`isCopilot` branches remain in bin/install.js,
install-engine.cts, surface.cts, or runtime-artifact-conversion.cts (AC2 guard scans all four).

UPGRADE 1 (multi-event hook bus): buildCopilotHookConfig() now emits preToolUse/
postToolUse/userPromptSubmitted/sessionEnd advisory handlers alongside sessionStart
(static inline bash/powershell — deterministic, golden-trackable). Only copilot.json's
gsd-session.json hash changes.

UPGRADE 2 (background dispatch): surfaced via the negotiated contract only —
dispatch.background:true exceeds the declarative-cli baseline and survives negotiation
with no downgrade warning. NO .agent.md frontmatter field (copilot has none). MCP
companion out of scope (AC4 names only 2 upgrades).

Tests: declarative-reference-copilot (adapter/axes/fail-closed + AC2 4-file source-grep
guard) + copilot-upgrades (live 5-event hook wiring; dispatch.background negotiation).
Matrix EoS note + how-to; changeset (Changed). capability-registry regenerated.

Incidental flaky-test RE-ARCHITECTURE (no-defer, maintainer-directed):
tests/opencode-review-reconstruction.property.test.cjs spawned ~600 synchronous
execFileSync('jq') subprocesses (numRuns:200 × 3 fast-check properties, one jq per
generated stream); a single jq freezing on a contended macos-22 CI runner hung the whole
unit-test chunk to its 600s kill (this PR's CI). --test-force-exit can't interrupt a
synchronous execFileSync, so the cure is to stop spawning per case, not just time-bound
it. Re-architected to run the SHIPPED jq program over the whole fast-check corpus in ONE
jq process: each generated stream is one compact-JSON array per line in a temp file,
`jq -c <PROGRAM>` (no -s) applies PROGRAM to each array (`.` == the array, exactly what
production's `jq -rs <file>` sees after slurping) and emits one result per line —
empirically byte-identical to the per-stream form across embedded-newline/empty/quote/
unicode/null-drop cases, and file-input (like production) so there's no stdin pipe to
deadlock on large I/O. ~600 spawns → 6; coverage unchanged (200-case corpus per property,
deterministic seeds) plus explicit boundary/diagnostic example batches. Still property-
tests the real shipped jq (no JS reimplementation). Per-call jq timeout retained as a
belt-and-suspenders bound.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/how-to/add-or-update-a-host-integration.md

@@ -147,6 +147,19 @@ yields the same truth value**, so behavior is unchanged and only the brittle cou
    the host correctly, negotiation fails closed on a corrupted descriptor, and — with a source-grep behind
    an `// allow-test-rule:` exemption — that **no `runtime === '<id>'` branch remains** in `bin/install.js`.
 
+**Another completed worked example: `copilot` (#2099).** Copilot was already installing through the
+declarative artifactLayout (not the direct `installRuntimeArtifacts` calls step 4 describes), so its
+migration folded the *residual* hardcoded branches rather than the whole install path: the `.agent.md`
+destination-suffix rename in `src/install-engine.cts` (→ `hostBehaviors.agentFileExtension`), two
+uninstall side-effect branches in `bin/install.js` (→
+`resolveInstallPlan(runtime).installSurface === 'copilot-instructions'`, already a live descriptor field
+elsewhere in the same file), and two `skipSharedHooksInstall` gates (→
+`hostBehaviors.skipSharedHooksInstall: true`). A dead legacy agent-converter dispatch arm — unreachable
+because copilot is a member of `_DESCRIPTOR_AGENTS_RUNTIMES` — was deleted outright rather than re-gated,
+mirroring step 6's guard: `tests/declarative-reference-copilot.test.cjs` source-greps both files for the
+retired `isCopilot` reads. See the `copilot` section of the reference matrix for the full EoS migration
+note, including the two upgrades (multi-event hook bus; negotiated `dispatch.background`) this PR adds.
+
 ---
 
 ## Related

docs/reference/host-integration-capability-matrix.md

@@ -461,6 +461,8 @@ Documentation gaps:
 - runtime — docs describe the CLI binary and the SDK (Node.js/Go/Python/Rust) but do not state what runtime the CLI host itself or its plugin/extension loader executes in.
 - dispatch.nested exact authoritative source is awesome-copilot.github.com (community docs) not docs.github.com.
 
+**EoS migration status (#2099):** Migrated onto the declarative adapter (dogfooded in `tests/declarative-reference-copilot.test.cjs`). The residual `isCopilot` branches were folded onto descriptor-driven `runtime.hostBehaviors`: the `.agent.md` destination-suffix rename in `src/install-engine.cts` now reads `hostBehaviors.agentFileExtension`; `bin/install.js`'s two uninstall side-effect branches (repo-root `AGENTS.md` cleanup, `copilot-instructions.md`/hook cleanup) now gate on `resolveInstallPlan(runtime).installSurface === 'copilot-instructions'` (unique to copilot, so byte-identical); and the two `skipSharedHooksInstall` checks now read `hostBehaviors.skipSharedHooksInstall:true` (copilot's golden has only `hooks/gsd-session.json`, no shared `gsd-*.js` scripts). A dead legacy agent-converter dispatch arm in the inline agent-copy loop — unreachable since copilot is a member of `_DESCRIPTOR_AGENTS_RUNTIMES` — was removed outright; `isCopilot` no longer appears as a live read anywhere in `bin/install.js` or `src/install-engine.cts`. Two upgrades land: (1) **multi-event hook bus** — `buildCopilotHookConfig()` previously emitted only `sessionStart`; this PR wires four additional events — `preToolUse`/`postToolUse`/`userPromptSubmitted`/`sessionEnd` — each a static, deterministic advisory command (no node-runner invocation), so an install's `hooks/gsd-session.json` now registers all five events. (2) **`dispatch.background`** — the descriptor already declared `true`, exceeding the `declarative-cli` profile baseline of `false`; the negotiation contract (`negotiateHostCapabilities`) surfaces that value with no downgrade warning, documenting the legitimate deviation. Note: Copilot's `.agent.md` frontmatter has no background-dispatch field (fields are `description`/`infer`/`mcp-servers`/`model`/`name`/`tools`) — background dispatch remains a negotiated-contract-only axis, not a field GSD's agent artifacts emit. MCP companion tooling is out of scope for this migration (AC4 names only the two upgrades above).
+
 ---
 
 ## kilo