F9: Terminal reassignment between threads (same project)

[rororowyourboat]

Summary

Allow users to move a terminal session from one Thread to another Thread within the same Project via a context menu action.

Important: Technical constraint

Terminals are in-memory only with composite keys `${threadId}\0${terminalId}` frozen at creation time in `Manager.ts`. There is no SQLite table for terminal sessions — only scrollback history on disk. The session map key cannot be changed after creation.

Live PTY reassignment (moving a running PTY without restart) requires an indirection layer that decouples the PTY process from the session key. This is a significant refactor.

Recommended v1 approach: close + reopen

Instead of live reassignment, v1 should:

1. Save terminal scrollback history
2. Close the terminal in the source Thread
3. Open a new terminal in the target Thread at the same cwd
4. Restore scrollback history in the new terminal

This preserves context (history) without requiring the Manager refactor.

Future v2: live reassignment

• Introduce TerminalProcess (owns PTY) separate from TerminalSession (owns key + UI binding)
• Allow re-keying a session to a different (threadId, terminalId) without killing the PTY
• This enables true drag-and-drop terminal movement

Scope (v1)

• Add context menu option on terminal tabs: "Move to thread..." → shows list of threads in same Project
• On selection: close terminal in source, open in target with same cwd, restore history
• Reject cross-project moves (different workspaceRoot)

Key files to modify

• apps/server/src/terminal/Layers/Manager.ts — add close + reopen sequence
• apps/server/src/terminal/Services/Manager.ts — add reassignTerminal method
• apps/web/src/components/ThreadTerminalDrawer.tsx — context menu with "Move to thread..."
• packages/contracts/src/terminal.ts — new RPC for reassignment if needed

Dependencies

None (independent of Client hierarchy, but P3 priority)

Acceptance criteria

• Context menu on terminal tab shows "Move to thread..."
• Shows only threads within the same Project
• Terminal closes in source, opens in target with same cwd
• Scrollback history preserved in target
• Cross-project moves rejected with message

[rororowyourboat]

Implemented in feat/sidebar-project-groups — terminal tab context menu with close+reopen approach.