TUI: add Harpoon-style pinned slots for direct agent switching

What variant of Codex are you using?

Codex CLI 0.150.1 (terminal TUI), using the /agents command center.

What feature would you like to see?

Add stable, user-assignable slots for active agent threads and direct keybindings to switch to them without reopening /agents each time.

Suggested interaction:

• In /agents, assign the highlighted agent to slot 1–9 (a StarCraft control-group / Harpoon-style model).
• From any agent thread, a configurable shortcut such as Alt+1 through Alt+9 switches directly to the existing thread in that slot.
• A direct shortcut such as Alt+0 returns to the parent thread.
• Show assigned slot numbers beside agents in the command center.
• Provide /agent 3 or an equivalent command fallback for terminals that cannot encode the configured chord.
• Keep slot assignments stable when agents move between Working, Needs input, and Ready groups.
• Switching must focus the existing live thread; it must not spawn or resume a duplicate.

The current repeated workflow is:

/agents → navigate or search → Enter → repeat whenever switching tasks.

With five to eight parallel agents whose responsibilities are already known—for example, “integration tests are slot 1”—the selector interrupts flow. tmux provides instant prefix + number navigation, but using separate tmux Codex sessions loses the native task status, lifecycle, steering, and result collection that make /agents reliable.

Pinned direct-access slots would preserve native Codex orchestration while making frequent agent switching a single action.

Additional information

Related to #22321, which introduced the Agent View. This is a narrower follow-up about fast repeated navigation among already-visible live agent threads, not a request for a global cross-process registry.

Useful acceptance criteria:

• One configurable action switches from any thread to an assigned agent.
• Slot mappings remain stable while dashboard ordering and status groups change.
• The parent thread has a direct binding.
• Selecting a slot never duplicates the agent thread.
• Terminal-safe command fallback is available when numeric modifier keys are unavailable.

Start with the /agents command center and the existing keyboard-action and command-dispatch entry points; trace how live agent threads, parent navigation, and status groups are identified. The work is done when configurable slots, direct switching, parent navigation, stable mappings, and a terminal-safe command fallback meet the listed acceptance criteria without duplicating threads.