fix(skills): invoke bundled Python helpers portably

[KumamuKuma]

Summary

Fix the four audited Python helper invocations in /understand-domain and /understand so they no longer depend on the caller's current working directory or on one platform-specific Python command.

This adds a shared Node launcher that:

• probes python, py -3, and python3 on Windows, and python3 and
  python elsewhere, from the trusted plugin directory;
• requires Python 3.10 or newer;
• resolves the interpreter to an absolute path before executing the helper;
• invokes subprocesses without a shell and preserves helper arguments and exit status; and
• works when the plugin or project path contains spaces or the plugin is installed through a symlink/junction.

The affected domain extraction, subdomain merge, and full/incremental batch merge commands now resolve both the launcher and helper through $PLUGIN_ROOT. Regression tests cover command snippets, platform selection, missing/incompatible interpreters, linked installs, paths with spaces, startup failures, and exit/signal propagation.

Linked issue(s)

None.

How I tested this

• corepack pnpm lint
• corepack pnpm --filter @understand-anything/core test — 970 passed
• corepack pnpm test — 488 passed, 15 skipped
• corepack pnpm exec vitest run --maxWorkers=2 — 488 passed, 15 skipped
• Targeted launcher and skill-command tests — 13 passed
• Python helper tests — 95 passed, 1 skipped
• Manual smoke test from a directory unrelated to the plugin and project, using a project path with spaces; domain-context.json was generated successfully
• Adversarial Windows smoke test with a fake python.exe in the analyzed project directory; the launcher ignored it and ran Python 3.13 from the trusted resolution path

Versioning

• Version bumped from 2.9.4 to 2.9.5 in all six authoritative manifests listed in CLAUDE.md
• N/A — internal/docs-only change