feat(#2097): migrate Augment onto EoS declarative adapter + settings.json MCP companion (ADR-1239)

Fold augment's runtime-literal conversion branches onto descriptor-driven
hostBehaviors and delete dead code:
- Site A (_applyRuntimeRewrites case 'augment'): the 4 ~/.augment dot-dir
  regexes now derive from getDirName('augment') via escapeRegExp (byte-
  identical; getDirName('augment')==='.augment') — no runtime literal.
- Site B (applyRuntimeContentRewritesForCommandsInPlace): the
  `if (runtime==='augment')` markdown-converter branch now reads
  runtime.hostBehaviors.commandBodyConverter and dispatches through a local
  COMMAND_BODY_CONVERTERS map (degrade-closed on unknown/absent name).
- Deleted dead `claudeToAugmentTools` map (zero refs; orphaned by ADR-1508
  single-sourcing) and the unreachable `else if (isAugment)` agent-conversion
  branch (augment ∈ _DESCRIPTOR_AGENTS_RUNTIMES → gated out upstream).
- Incidental orphan cleanup (no-defer): removed the equally-unreachable
  `else if (isTrae)` agent-conversion arm left behind by trae's already-merged
  migration #2094 (trae ∈ _DESCRIPTOR_AGENTS_RUNTIMES, same upstream gate).
  copilot/windsurf/codebuddy arms are removed by their own pending migrations.

UPGRADE 3 (transport:mcp): register the GSD companion MCP server in Augment's
settings.json under mcpServers.gsd (Augment hosts MCP in settings.json, not a
standalone file). mergeGsdMcpServerIntoSettings mutates the in-memory settings
object finishInstall already writes (gated on hostBehaviors.mcpCompanion===
'settings-json'); non-destructive + idempotent; symmetric uninstall removal.
settings.json is golden-excluded, so no golden change. UPGRADE 1 (named/
background dispatch) + UPGRADE 2 (settings-json hook bus, Claude dialect)
were already live in production — this adds tests exercising both.

Golden: byte-identical for all 16 runtimes (folds preserve regex behavior;
MCP lives in golden-excluded settings.json) — verified by a real double-install
tree diff. Tests: declarative-reference-augment (adapter/axes/fail-closed/
undocumented-sub-axes + source-grep guard scoped to conversion-logic branches)
+ augment-upgrades (dispatch negotiation, hook-bus live install, MCP add/
idempotent/preserve/uninstall). Matrix + connect-gsd-mcp-server + a stale
install-on-your-runtime hook-ownership claim corrected; changeset (Changed).

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

.changeset/2097-eos-augment-imperative-adapter.md

@@ -0,0 +1,6 @@
+---
+type: Changed
+pr: 2166
+---
+
+**Augment Code now installs through its capability descriptor, with a native MCP companion** — installing GSD into Augment registers the GSD companion server in Augment's `settings.json` `mcpServers` and drives command/skill/agent conversion from Augment's negotiated descriptor instead of hardcoded runtime special-cases. (#2097)