Show live Git diff and PR status in Herdr worktree Spaces

[leesangb]

Summary

• parse Herdr 0.7.4 event envelopes so worktree PR and CI metadata watchers start correctly
• report tracked line additions/deletions and untracked file counts as the $diff workspace token
• refresh Git diff metadata immediately on focus, every 30 seconds while focused, and every five minutes in the background
• keep Git diff and GitHub watchers independently locked and expire stale metadata
• document a dedicated $diff sidebar row to avoid truncation after long branch names

Verification

• bun test (236 tests)
• bun run typecheck
• bun run build:herdr-plugin
• exercised the compiled watcher against an open linked worktree Space

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