feat(keybindings): add mod+. shortcut to cycle agent trait

[JtMotoX]

What Changed

Add an agent.cycle keybinding command with a default mod+. binding (when not in terminal focus). Pressing it cycles through the available agent options (e.g. Build, Plan) for the current provider model, wrapping from last to first. No-ops when fewer than 2 agents are available.

The traits dropdown now displays the agent.cycle shortcut label next to the "Agent" section header, so users can discover the hotkey without memorizing it.

Changes across 5 files (+55 lines):

• packages/contracts/src/keybindings.ts — add agent.cycle to static keybinding commands
• apps/server/src/keybindings.ts — add default mod+. binding
• apps/web/src/components/ChatView.tsx — handle agent.cycle in the existing keydown handler
• apps/web/src/components/chat/TraitsPicker.tsx — show the agent.cycle shortcut label (e.g. ⌘.) in the traits dropdown Agent section header
• apps/web/src/keybindings.test.ts — add test coverage for the new binding and its label rendering

Why

OpenCode uses mod+. to cycle agents. T3 Code supports agent selection via the traits dropdown but has no keyboard shortcut for it. This adds parity for keyboard-driven workflows and makes the shortcut discoverable in the UI.

Closes #2299

Screenshots

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes (N/A)

Note

Add mod+. shortcut to cycle agent trait in the composer

• Adds agent.cycle to the static keybinding commands and maps mod+. to it (when the terminal is not focused) in keybindings.ts.
• In ChatView.tsx, handling agent.cycle computes the next agent from available provider/model capabilities in a cyclic order and persists the selection with persistSticky=true.
• The Agent section header in TraitsPicker.tsx now displays the active shortcut label (e.g. ⌘. on Mac, Ctrl+. on Linux) using the Kbd component.

^{Macroscope summarized ea81efa.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new keyboard shortcut feature (mod+. to cycle agent traits) with new user-facing behavior and state persistence logic. New features benefit from human review to validate the interaction design and ensure the cycling logic behaves as expected across different providers and configurations.

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