fix: remaining audit findings not yet fixed on main (release 0.8.13)

[TMHSDigital]

Summary

Supersedes #54, which was based on 0.2.2 — most of its findings were independently fixed on main between 0.2.2 and 0.8.12 (#25, #27, #28, #32, #43). This PR carries over only the fixes that are still missing on current main:

• Crashed recordings are recorded as failed, not stopped. The child-exit handler now checks the exit code and stores the stderr tail; the failed status existed in the type but was never assigned. A deliberate stop_recording still ends as stopped (it clears the transient failure marker set when a stop has to force-kill).
• concat works with relative input paths. The demuxer resolves list entries against the list file's directory (the OS temp dir since fix: write temp working files to the OS temp dir, not the edits dir (#24) #30), not the cwd the inputs were validated against; inputs are now resolved to absolute first.
• The server advertises its real version. index.ts hardcoded 0.8.0 while the package is at 0.8.12; the version is now read from package.json at runtime.
• A corrupt sessions.json is backed up to .bak (with a stderr note) instead of silently discarded along with any live-recording entries it held.
• stop_recording's blind 400 ms flush sleep is replaced with pid-exit polling (kill paths) or an immediate probe (graceful path).
• CLAUDE.md no longer references a src/providers/ layer that doesn't exist in this codebase.

Version bumped to 0.8.13 with a CHANGELOG entry.

Testing

• npm run build, npm run typecheck, npm run lint, npm test — 148 passed / 6 skipped (ffmpeg-gated suite), including a new regression test for the corrupt-registry backup.
• Runtime smoke over MCP stdio against real ffmpeg 8.1.1, all passing: force-killed ffmpeg → session failed with error tail; graceful stop → stopped with duration probed in ~184 ms (previously a fixed 400 ms floor); concat with relative input paths; advertised server version matches package.json (0.8.13).

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