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]>

docs/GETTING_STARTED.md

@@ -11,13 +11,19 @@ For the windows installation, please see [windows_installation](windows_installa
 
 ## 1. Install
 
+Install the pinned Node toolchain first — Node 22.12 or newer (see
+`.nvmrc`; with nvm: `nvm install && nvm use`). Installs are refused on
+older Nodes.
+
 From the repository root:
 
 ```bash
-npm install
+npm ci
 ```
 
-That installs Electron and the local packaging tools used by the scripts.
+That installs the locked dependency tree — Electron and the local packaging
+tools used by the scripts. `npm ci` is the only supported installation path;
+the app never installs or repairs dependencies at runtime.
 
 ## 2. Launch the app