feat(#2002): self-healing runtime build for plugin-marketplace installs (#2036)

The compiled gsd-core/bin/lib/*.cjs modules are gitignored build artifacts
(ADR-457) shipped prebuilt in the npm tarball. A Claude Code plugin-marketplace
/ git-clone install materializes the repo tree directly and never runs
`npm run build:lib`, so those ~148 files are absent and every CLI command dies
at load with `Cannot find module './lib/cli-exit.cjs'`.

gsd-tools.cjs now calls ensureRuntimeBuild() before its ./lib requires: a new
committed bin helper that compiles the tree once on demand (lock-guarded,
incremental-cache-cleared, portable `node <tsc>` invocation) when the sentinel
cli-exit.cjs is missing, and surfaces an actionable error when TypeScript is
unavailable. The already-built npm path is a single fs.existsSync no-op.
ADR-457 (gitignored, build-at-publish) is preserved; no release-pipeline change.

Verified against a simulated marketplace checkout (148 build outputs removed):
the CLI self-heals and the result is byte-identical to `npm run build:lib`.

Closes #2002

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

tests/fixtures/golden-install-parity/opencode.json

@@ -107,7 +107,8 @@
   "command/gsd-workstreams.md": "5e57eed1881c3891",
   "gsd-core/VERSION": "ef0deccd81a6723c",
   "gsd-core/bin/check-latest-version.cjs": "e4a224058c8f4d74",
-  "gsd-core/bin/gsd-tools.cjs": "e504d3b82da834ae",
+  "gsd-core/bin/ensure-runtime-build.cjs": "51bc64467ab30f62",
+  "gsd-core/bin/gsd-tools.cjs": "770e0b1c1f08ef47",
   "gsd-core/bin/gsd_run": "62d9b647ede212e6",
   "gsd-core/bin/shared/config-defaults.manifest.json": "517e6a7c1e9f4f16",
   "gsd-core/bin/shared/config-schema.manifest.json": "c55b99e5d82f69fb",