Unify agent sessions and redesign the model picker

[gaoyu06]

Agent sessions are one tab type. TUI/ACP are no longer listed as create-tab options; the coding agent is chosen inside a session. Composer no longer uses a ⋯ overflow: model click opens a popup (agent rail when unlocked, then models with hover thinking chips), voice sits next to a smaller circular send, and approval / context / git branch live in a footer strip under the card.

Scope

• Mosaic + / empty canvas / palette / sidebar: only “创建 Agent 会话”. Persisted tui:* tabs still render; they are not offered as new options.
• Model button opens AgentModelPopover: left = coding-agent icons (hidden once the session is locked after first message or restore); center = that agent’s models; hover a model to pick a thinking level (chips, not a slider).
• Composer bar: attach, model, voice, circular send. Footer outside the card: git branch + approval mode (left), context ring + text (right).

Ship-ready?
Not yet. Unit tests are green; GUI walkthrough and review are still outstanding on this draft.

Needs review?
Yes — especially backend switch from the popover rail (does the picker survive ChatState swap?), locked vs unlocked rail, and that TUI/ACP cannot be created from any remaining menu.

Review preconditions

• Branch is cursor/agent-session-composer-17c0 on current main (workspace tabs already merged).
• pnpm test / pnpm check should be green (464 tests at last run).
• Manual: + menu has no TUI rows; new session can switch JuCode/Claude/Codex before the first message and cannot after; hover-effort selects model+effort; footer shows branch + context.

Test results
pnpm test: 29 files, 464 tests passed. pnpm check: 0 errors.