fix: move expensive operations off UI event loop and fix stale preview pane

[miamachine]

Summary

Three related fixes that improve Claude Squad UI responsiveness and fix stale preview state:

1. Move metadata updates off the UI event loop

• Root cause: The tickUpdateMetadataMessage handler runs git add -N . + git diff for every active instance synchronously on the Bubble Tea event loop. On large repos (225K files / 30GB), each instance takes ~2.6s, so 5 instances blocks all input processing for ~13 seconds per tick.
• Fix: Move the per-instance metadata work (HasUpdated + diff computation) into a tea.Cmd that runs in a background goroutine, and parallelize across instances with a sync.WaitGroup. Results are sent back as a metadataUpdateDoneMsg and applied on the main goroutine to avoid data races with View.
• Guard against pile-up: A metadataUpdating flag prevents overlapping ticks from spawning concurrent updates.

2. Move session creation off the UI event loop

• Root cause: instance.Start(true) (git worktree setup, tmux session creation, trust screen detection) blocks the bubbletea Update handler for 30-60+ seconds, especially with CrowdStrike.
• Fix: Run instance.Start(true) as an async tea.Cmd with a Loading status for visual feedback (spinner). Guards prevent user interaction with not-yet-started instances.
• Bonus: Fixes a bug where newInstanceFinalizer() was called twice per session creation, double-counting the repo in the multi-repo display.
• Credit: Jacob Massey

3. Refresh preview pane after Ctrl+Q detach

• Root cause: After pressing Ctrl+Q to detach from an attached tmux session, the onDismiss callback set m.state = stateDefault but never called m.instanceChanged(), so the preview pane stayed frozen showing stale content from before the attach.
• Fix: Add m.instanceChanged() call after detach, matching the pattern used by every other showHelpScreen callback (checkout, kill, etc.).

Impact

• UI blocking drops from ~13s to near-zero during metadata ticks
• Session creation no longer freezes the UI
• Preview pane correctly refreshes after returning from an attached session

Test plan

• go build ./... compiles cleanly
• Manual testing: run cs with 5+ instances on a large repo, verify input is responsive
• Verify instance status indicators (Running/Ready) still update correctly
• Verify diff stats in the diff tab still update correctly
• Verify AutoYes (TapEnter on prompt) still triggers correctly
• Create a new instance, verify spinner shows during startup and UI stays responsive
• Attach to a session (Enter), detach (Ctrl+Q), verify preview updates immediately

🤖 Generated with Claude Code

[github-actions]

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❌ @miaemyle
❌ @jacobmassey
❌ @mia hebert
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[miamachine]

I have read the CLA Document and I hereby sign the CLA

(code was co-written with Claude, and this human reviewed the changes thoroughly and tested manually as noted above -- for more context, I've been using this tool with an extremely large monolith and the update loop was making the tool practically unusable for my needs)

[miamachine]

recheck

[mufeez-amjad]

@miamachine Thanks for the PR!

I think you can simplify this – this should work with just one self-chaining Cmd:

func tickUpdateMetadataCmd(instances []*session.Instance) tea.Cmd {
    return func() tea.Msg {
        time.Sleep(500 * time.Millisecond)
        // filter active, spawn goroutines, wg.Wait()
        return metadataUpdateDoneMsg{results: results}
    }
}

[jayshrivastava]

Similar PR: #249

[miamachine]

done @mufeez-amjad !

merged the sleep + filtering + goroutines + wg.Wait() all into a single self-chaining tickUpdateMetadataCmd(instances []*session.Instance) tea.Cmd. This eliminates the intermediate tickUpdateMetadataMessage type, the metadataUpdating guard flag, and the two-step dispatch. Much cleaner — thanks for the suggestion.

[miamachine]

recheck

[miamachine]

re: CLA idk why but my username is weird -- @miaemyle and @miamachine point to the same user though not sure why only the former shows up (which might be an issue for the bot to approve this PR)