Address review feedback in uninstall JSON, guarded execution, and context provenance

[Copilot]

This follow-up addresses the actionable bot feedback on the release PR. The changes tighten machine-readable failure reporting, make guarded timeouts clean up spawned processes correctly, and remove generated dev.kit artifacts from manifest provenance.

• Uninstall JSON contract

  • capture bin/scripts/uninstall.sh exit status in JSON mode
  • return ok: false with an error field on failure instead of always reporting success
  • preserve binary_removed / home_removed from actual post-run state

• Guarded timeout behavior

  • terminate descendant processes, not just the top-level PID, when a guarded command exceeds the hard timeout
  • escalate from TERM to KILL after a short grace period
  • keep timeout handling bounded even when the command spawns background work

• Repo context provenance

  • exclude generated .rabbit/dev.kit/* artifacts from contract-evidence scanning
  • regenerate .rabbit/context.yaml so used_by / evidence only reflect repo-owned sources

• Repo write error reporting

  • distinguish timeout failures (124) from other context write failures
  • surface the actual non-timeout exit status instead of reporting every failure as a timeout

• Suite portability and coverage

  • replace the Perl-based context mutation in /home/runner/work/dev.kit/dev.kit/tests/suite.sh with a portable shell/awk helper
  • add coverage for uninstall failure JSON, timeout child cleanup, non-timeout repo write failures, and generated .rabbit/dev.kit/* exclusion

# before
dev.kit --json uninstall --yes
# => { "command": "uninstall", "ok": true, ... }  # even if uninstall failed

# after
dev.kit --json uninstall --yes
# => { "command": "uninstall", "ok": false, "error": "...", ... }