feat(#2088): migrate Codex onto the Embeddable Orchestration System (ADR-1239)

Drive Codex install/uninstall through the descriptor-driven Host-Integration
Interface (declarative embedding adapter → engine surface dispatch) and fold
every positive `runtime === 'codex'` / `isCodex` projection into descriptor-driven
`runtime.hostBehaviors`. Install/uninstall output stays byte-parity-gated
(tests/fixtures/golden-install-parity/codex.json); no other runtime changes.

Three Context7-verified upgrades, each with a test on the user-reachable surface:
- Skill root → canonical $HOME/.agents/skills via a skills-kind `home` override,
  with pre-move migration cleanup (stale ~/.codex/skills/gsd-* removed on install
  and uninstall; user content preserved). Fixes getGlobalSkillsBase, writeManifest,
  and the skill-manifest inventory to honor the override so --skills-root /
  sync-skills / the manifest report the real location.
- Six new hooks.json lifecycle events (PreToolUse, PermissionRequest, PreCompact,
  PostCompact, SubagentStop, UserPromptSubmit) shared by install + uninstall;
  extendedHookEvents reconciled [] -> the schema-valid wired subset.
- Explicit `[agents] max_depth = 1` in the managed config.toml block, pinning the
  negotiated dispatch.maxDepth:1 axis. validateCodexConfigSchema now permits a
  known-scalar-only bare `[agents]` AgentsToml table (still rejects [[agents]] and
  unknown-key break-forms, #2760); mergeCodexConfig preserves the user's own
  AgentsToml scalars (max_threads etc.) instead of dropping them.

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

.changeset/2088-eos-codex-declarative-adapter.md

@@ -0,0 +1,4 @@
+---
+type: Changed
+---
+**Codex is now driven through the public Host-Integration Interface, with three capability upgrades (ADR-1239 / EoS).** Codex previously installed via hardcoded `runtime === 'codex'`/`isCodex` projection in `bin/install.js`; its `config.toml` / agent-`.toml` / `hooks.json` install now runs through the declarative embedding adapter and descriptor-driven `runtime.hostBehaviors`, with **zero** positive `isCodex` gates and **zero** `runtime === 'codex'` branches remaining (source-guarded). Install/uninstall output stays byte-parity-gated (`tests/fixtures/golden-install-parity/codex.json`). Three Context7-verified upgrades land, each with a test driving the user-reachable surface: (1) **skill root** — GSD skills now install to Codex's canonical `$HOME/.agents/skills` (via a skills-kind `home` override) instead of the deprecated `$CODEX_HOME/skills` fallback, and pre-move installs are migrated (stale `~/.codex/skills/gsd-*` cleaned on both install and uninstall, user-owned content preserved); (2) **hook events** — GSD registers the six documented Codex lifecycle events it previously skipped (`PreToolUse`, `PermissionRequest`, `PreCompact`, `PostCompact`, `SubagentStop`, `UserPromptSubmit`, in addition to the existing `SessionStart`/`SubagentStart`/`Stop`/`PostToolUse`) in `hooks.json`, so `gsd-context-monitor` fires at the same points as in Claude Code, and the descriptor `extendedHookEvents` is reconciled from `[]` to the schema-valid wired subset; (3) **dispatch tuning** — `[agents] max_depth = 1` is written explicitly into the managed `config.toml` block to pin the negotiated `dispatch.maxDepth: 1` axis (`degradationFor` flattens GSD-hosted waves to single-level), and `validateCodexConfigSchema` now permits a known-scalar-only `[agents]` AgentsToml table (coexisting with the flattened `[agents.gsd-*]` role sub-tables) while still rejecting the `[[agents]]` and unknown-key break-forms from #2760. (#2088)