fix: pane-centric migration cleanup - normalize mixed session keys

[boxp]

Summary

• Add read-time normalization (normalize-sessions) to read-state-file that re-keys legacy session-id entries to pane-id canonical keys
• When multiple entries share the same pane-id, the entry with the latest :last-updated wins
• Write operations (which read first) automatically clean up the file on next write
• Ensures 1 pane = 1 row even with mixed-key sessions.edn from pre-migration period

Root Cause

After pane-centric migration, sessions.edn can contain both session-id keys (UUIDs) and pane-id keys (%NN) for the same pane, causing duplicate entries in the TUI.

Canonicalization Strategy

• Entries with non-empty :pane-id -> keyed by pane-id
• Entries without :pane-id -> keep original key (non-tmux sessions)
• Conflicts resolved by :last-updated timestamp (newest wins)

Cleanup Behavior

• Read-time: read-state-file normalizes on every read (transparent to callers)
• Write-time: All write functions read first, so next write produces a clean file

Tests (7 new)

1. Mixed session-id + pane-id key normalization
2. Newer timestamp entry wins regardless of key type
3. Different pane-ids stay as separate entries
4. Non-tmux sessions (empty pane-id) keep session-id key
5. Multiple legacy session-id keys for same pane collapse to one
6. Read-then-write produces clean file
7. Update after normalize leaves no legacy key remnants

Review Points

• Migration logic is minimal (single reduce-kv pass)
• No dead compatibility code added
• pane-centric canonical store assumption is consistent
• Tests cover the exact mixed-state scenario from the bug report

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 4a9877a243

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".