Change agent-cycle shortcut from Cmd+. to Shift+Tab

[jeonghun-jj-lee]

Important

Problem — The agent-mode cycle shortcut (Cmd+.) collides with system/editor conventions and is not discoverable. Shift+Tab is a more natural, single-hand shortcut for rotating through modes.
Approach — Change the agent.cycle keybind from "mod+." to "shift+tab" and disable the reverse-cycle keybind (agent.cycle.reverse). Tooltips auto-update from the command registration — no UI code changes required.
Scope — in: forward-cycle keybind, reverse-cycle disable, tooltip display · out: command palette entry (unchanged), slash command /agent (unchanged), i18n strings (unchanged)

Acceptance Criteria

• Pressing Shift+Tab anywhere in the session (including while the composer is focused) cycles the agent forward through the configured order
• The tooltip on the agent selector displays the new shortcut (⇧Tab on macOS)
• The reverse-cycle shortcut (Shift+Cmd+.) no longer fires
• Cmd+. no longer cycles agents
• The "Cycle agent" entry in the command palette shows the updated keybind

Key Decisions

• Forward only. The reverse-cycle keybind is disabled ("none"), not removed — the command stays registered for palette/slash access. Cycling wraps, so reverse is low-value.
• Shift+Tab, not plain Tab. Plain Tab conflicts with focus navigation; Shift+Tab is a clean modifier chord with no standing duty in the composer.

Constraints & Invariants

• The command.tsx keydown handler's existing isTab guard already allows Tab-key shortcuts through the editable-target filter — that logic must not regress.

Prior Art

• use-composer-commands.tsx — the command registration module in the opencode fork
• command.tsx — the keybind parser and keydown handler (overlay)

Notes

• Single-file change in the fork (use-composer-commands.tsx), then pnpm --filter amicode opencode:build to re-vendor.