Fix merged PR matching and auto-clean merged worktrees

[webdevcody]

Summary

• Validate branch PR lookups against checkout branch, default branch, and PR head SHA so stale merged PRs from long-lived branches (like main) do not make current checkouts purple.
• Add opt-in auto_cleanup_merged under Settings > Sessions to clean safely merged linked worktrees.
• Add daemon-side cleanup safety checks for root/default branches, branch/head drift, dirty or untracked worktrees, active sessions, and running terminals, plus docs and tests.

Tests

• cargo +stable test -q -p nebula-tui pull_request --no-default-features
• cargo +stable test -q -p nebula-tui auto_cleanup --no-default-features
• cargo +stable test -q -p nebula-daemon cleanup_merged_worktree --no-default-features
• cargo +stable check -q --workspace --no-default-features

Notes

• cargo +stable test -q --workspace --no-default-features was stopped after the existing tui_drag_past_the_pane_top_autoscrolls_and_copies_the_run test hung for several minutes locally; earlier test groups in that run had passed before the hang.
• The initial Ubuntu CI failure was a test setup issue (temp git repo missing author identity) and is fixed in the latest push.

  

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.