Pin the Node toolchain, remove runtime npm repair, make CI and E2E truthful

Phase 0 of the improvement plan (ai_prompts/prompt4.md): make the baseline
reproducible and stop the test runner from masking real failures.

- Pin Node >= 22.12 (engines + .nvmrc + engine-strict); every entry point
  fails fast with clear guidance instead of dying late with ERR_REQUIRE_ESM
  inside the packaging dependency graph.
- electron-launcher.js is diagnostics-only: all runtime npm install/rebuild/
  repair paths are removed. npm ci on the pinned toolchain is the only
  supported install path (README + GETTING_STARTED updated).
- Refuse to silently launch unsandboxed on Linux: --no-sandbox now requires
  an explicit STEPFORGE_ALLOW_NO_SANDBOX/ELECTRON_DISABLE_SANDBOX opt-in and
  is otherwise a hard error with actionable fixes; user-namespace sandboxing
  is detected and preferred.
- Click-capture E2E no longer converts startup crashes into "SKIPPED": the
  only allowed skip is the upfront absence of a display server. A missing
  shared library or crash now fails with the startup log. Same guard added
  to the startup smoke check.
- GitHub CI: run on pull_request, pin Node from .nvmrc, drop the macOS matrix
  entry (not a support target), and audit production and full dependency
  trees as separate signals. Gitea CI: pull_request trigger + pinned Node.
- Refresh package-lock on Node 22/npm 10 and remediate the form-data and
  undici advisories (npm audit: 0 vulnerabilities, prod and full tree).
- Stop tracking generated machine-specific build reports
  (build/build_report.md, build/artifacts_manifest.json).

Verified: 203 unit tests pass; repo-structure, startup-smoke, unit-workflows,
sample-artifacts, and build-release checks pass locally with a real Electron
launch. The click self-test now truthfully reports the pre-existing Linux
arm/debounce capture failures (also red on Gitea CI main run 177) instead of
hiding behind SKIPPED; that defect is scheduled for the capture-fix PR.

Co-Authored-By: Claude Fable 5 <[email protected]>

README.md

@@ -66,13 +66,17 @@ For a Windows installation, see [docs/windows_installation](docs/windows_install
 
 On **Linux** (⚠️ work in progress — X11 vs Wayland, enabling per-click capture, the screen-share prompt), see [docs/GETTING_STARTED_WITH_LINUX.md](docs/GETTING_STARTED_WITH_LINUX.md).
 
-Requirements: Node.js 20+ and npm (Electron is the only dependency).
+Requirements: Node.js 22.12+ and npm (pinned in `.nvmrc`; installs are
+refused on older Nodes because the packaging toolchain needs 22.12+).
 
 ```bash
-npm install        # one-time, fetches the Electron shell
+npm ci             # one-time, installs the locked dependency tree
 npm start          # launch StepForge
 ```
 
+Dependencies are only ever installed by you, via `npm ci` — the app never
+downloads or repairs packages at runtime.
+
 First run creates the local data directory (`~/.local/share/stepforge` on
 Linux (WIP), `%APPDATA%/stepforge` on Windows; override with
 `STEPFORGE_DATA_DIR`).