Prune stale and duplicate Git delivery candidates

[bhavsarpratik]

Summary

• discard deleted, non-directory, and non-Git roots before the repository cap
• deduplicate identical working directories before invoking Git
• coalesce linked worktrees using validated common-directory identity
• migrate existing linked-worktree ledger evidence without double-counting
• keep dashboard reads Git-free through a race-safe watcher-owned snapshot
• retain the 50+1 overflow sentinel and PR Fix Git delivery watcher interval floor #31's five-minute scan floor

Fixes #33.

Verification

• 71 Git-delivery tests passed
• Python compilation and git diff --check passed
• installed-runtime parity, /health, /menubar, /git-delivery, and both LaunchAgents passed
• independent tester and two independent review lenses passed with no findings

Full suite: 925 tests run, 16 skipped, with one pre-existing Pi README wording failure.

Relationship to #31

This branch retains the monotonic scan-floor behavior from #31 because both fixes touch the watcher. If #31 lands first, this PR will be rebased before merge.