refactor(#1734): extract install engine from bin/install.js (ADR-1239 Phase B deep move) (#1735)

* refactor(#1734): extract install engine from bin/install.js (ADR-1239 Phase B deep move)

Relocate the runtime-artifact install cluster out of the 12,490-line
bin/install.js into a dedicated src/install-engine.cts -> install-engine.cjs:
installRuntimeArtifacts, uninstallRuntimeArtifacts, installOpencodeFamilySkills,
and their cluster helpers (_copyStaged, snapshot/restore, legacy migration,
GSD-entry pruning, preserve/restoreUserArtifacts, OpenCode-family converters,
USER_OWNED_ARTIFACTS).

- bin/install.js imports the engine and re-exports the moved symbols for
  back-compat; getCommitAttribution STAYS in install.js (impure config I/O +
  argv explicitConfigDir global) and is injected via a resolveAttribution param.
- 17 test files migrated to import the moved symbols from the engine.
- Bookkeeping: eslint built-artifact ignore, .gitignore, INVENTORY manifest+row,
  CONTEXT.md Install Engine Module glossary seam.

Behaviour-preserving: install output is byte-identical for all 16 runtimes
(golden-parity harness #1730) — the only delta is the new install-engine.cjs
file shipping in the installed gsd-core/bin/lib/ tree.

Closes #1734

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

* chore(#1734): backfill changeset PR number (#1735)

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

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: review-bot <review-bot@gsd>

tests/install-regressions.test.cjs

@@ -37,7 +37,12 @@ try {
   else process.env.GSD_TEST_MODE = savedTestMode;
 }
 
-const { install, installRuntimeArtifacts, uninstallRuntimeArtifacts, mergeClaudePermissions, GSD_CLAUDE_ALLOW_PERMISSIONS, GSD_CLAUDE_DENY_PERMISSIONS, rewriteLegacyManagedNodeHookCommands, resolveNodeRunner } = installExports || {};
+const { install, mergeClaudePermissions, GSD_CLAUDE_ALLOW_PERMISSIONS, GSD_CLAUDE_DENY_PERMISSIONS, rewriteLegacyManagedNodeHookCommands, resolveNodeRunner } = installExports || {};
+
+const {
+  installRuntimeArtifacts,
+  uninstallRuntimeArtifacts,
+} = require('../gsd-core/bin/lib/install-engine.cjs');
 
 const INSTALL_SCRIPT = path.join(__dirname, '..', 'bin', 'install.js');
 const HOOKS_SRC = path.join(__dirname, '..', 'hooks');