fix: make phase 7 cleanup reversible

[sjh9714]

Summary

Fixes #301.

Phase 7 no longer asks the host shell to immediately delete freshly-created .understand-anything/intermediate and tmp work directories. Instead, it calls a bundled cleanup script that moves those directories into timestamped .trash-* directories and only purges trash older than seven days.

Changes

• Add cleanup-workdirs.mjs to move intermediate/ and tmp/ into reversible trash and prune expired trash.
• Update /understand Phase 7 to call the bundled cleanup script.
• Add regression coverage for moving work dirs, retaining recent trash, purging expired trash, and avoiding the old Phase 7 cleanup command.
• Bump plugin manifest versions to 2.7.6 per the repository versioning note.

Testing

• pnpm install
• node --check understand-anything-plugin/skills/understand/cleanup-workdirs.mjs
• pnpm vitest run tests/skill/understand/test_cleanup_workdirs.test.mjs
• pnpm lint
• pnpm test
• pnpm --filter @understand-anything/core test
• pnpm --filter @understand-anything/skill build
• pnpm build
• git diff --check

Note: I used Codex while preparing this change, reviewed the final diff, and ran the listed checks locally.

[sjh9714]

Rebased this onto the latest main and resolved the Phase 7 cleanup conflict by keeping the shared cleanup script path while preserving the upstream scan-result.json behavior.

Verified locally:

• pnpm test tests/skill/understand/test_cleanup_workdirs.test.mjs
• pnpm lint
• pnpm test
• pnpm build
• git diff --check