fix(agent-chat): preserve draft text when pane splits

[justinchoo93]

Summary

• Pane splits restructure the React component tree, causing ChatComposer to unmount/remount and lose typed text from useState
• Add a module-level Map<paneId, string> draft store (src/lib/draft-store.ts) that survives React lifecycle — ChatComposer reads from the store on mount and syncs on every keystroke
• Drafts are cleaned up on pane close (PaneContainer) and tab close (tabsSlice)
• Handles edge case where paneId prop changes in-place (resyncs text + textarea height)

Test plan

• Unit tests for draft-store (get/set/clear, independent per paneId, empty string cleanup)
• Unit tests for ChatComposer draft preservation (restore after remount, clear on send, independent per paneId, no-paneId fallback)
• Manual: type text in a Freshclaude pane → split the pane → verify text is preserved
• Manual: type text → send message → verify draft is cleared
• Manual: close a pane with draft text → reopen → verify draft is not leaked

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[chatgpt-codex-connector]

Clear draft when closing panes after content replacement

This cleanup only runs when the pane is currently agent-chat, but drafts are keyed by paneId and survive content switches. If a user types a draft, replaces that pane with another kind via updatePaneContent, and then closes the pane, this branch is skipped and the old draft remains in the module-level map indefinitely. That creates a leak over repeated replace/close cycles and can surface stale text if that pane id is reused; draft cleanup should also happen on pane close by paneId regardless of current content or when replacing away from agent-chat.

Useful? React with 👍 / 👎.