refactor(3664): migrate bin/install.js install/uninstall to Runtime Artifact Layout Module (#3674)

Phase 2 of #3660 / ADR-3660. Routes both lifecycle verbs through the
Runtime Artifact Layout Module landed in Phase 1 (#3663):

- Add installRuntimeArtifacts(runtime, configDir, scope, resolvedProfile)
  and uninstallRuntimeArtifacts(runtime, configDir, scope) as the public
  orchestrators. Both pre-prune stale gsd-* entries before staged copy;
  installRuntimeArtifacts brackets the prune+copy with preserveUserArtifacts /
  restoreUserArtifacts so user-owned content (e.g. gsd-dev-preferences) survives
  wipe-and-replace for claude/qwen/hermes runtimes.
- Add applyRuntimeContentRewritesInPlace as the per-runtime path/branding
  post-stage step (preserves byte-output equivalence with the legacy
  copyCommandsAs* pipeline, including Qwen/Hermes branding rewrites).
- Add _copyStaged, _removeGsdEntries kind-aware filesystem helpers.
- Add _runLegacyInstallMigrations, _runLegacyUninstallCleanup as thin
  dispatchers over existing ADR-0008 legacy migrations (Hermes flat->nested
  per #2841, dev-preferences-as-skill per #2973). For Hermes, also clean up
  the intermediate skills/gsd/gsd-*/ layout that pre-Phase-2 installs left
  on disk.
- Delete the 9 copyCommandsAs*Skills functions (Codex / Cursor / Windsurf /
  Trae / CodeBuddy / Copilot / Claude / Antigravity / Augment) and the
  _copyCommandsAsSkillsViaConverter helper. All test entry points migrated
  to call installRuntimeArtifacts directly through the unified seam.
- Collapse the 9-branch uninstall ladder to one uninstallRuntimeArtifacts
  call plus preserved non-layout side-effects (Codex TOML, Copilot
  instructions, hooks).
- Unify install dispatcher: a single _isSkillsRuntime gate routes all 11
  skills runtimes through installRuntimeArtifacts for both full and core/
  minimal profiles. Removes 11 per-runtime if-else branches (3 minimal-mode
  shim branches + 8 dead after-the-gate branches).

Net delta on bin/install.js: 11,495 -> 11,174 (-321 LOC).

New tests:
- tests/install-uninstall-layout-loop.test.cjs (34 tests) - per-runtime
  fixture assertions on install/uninstall/legacy-migration ordering.
- tests/install-hermes-regressions.test.cjs (6 tests) - covers the six
  defects surfaced by iterative review: Hermes upgrade leaves stale dirs,
  --hermes --profile=core fall-through, --qwen --profile=core fall-through,
  minimal-mode dev-preferences migration skipped (Hermes/Qwen/Claude-global),
  and ordering bug in _runLegacyInstallMigrations.

Existing tests (10,038 prior + 40 new) all green: 10,078/10,078 pass.

Refs #3664

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>