fix(desktop): declare a mode for Bot sessions

[orangeCatDeveloper]

Fixes #4193

Every Bot conversation (Feishu, Telegram, WeCom) answers Maka 暂时无法处理这条消息：机器人对话处理失败 and no session is ever created, while the platform connection itself stays healthy.

Root cause: explore is a boundary a product mode confers, not one a caller may request directly (create-session-input.ts), but the Bot adapter requested it directly — permissionMode: 'explore' with no mode. prepareCreate rejects that, and the rejection matches no category in generalizedErrorMessage, so the Bot shows the generic fallback and the failure reads like a platform or credentials problem. Desktop chat is unaffected: it starts at ask.

A Bot session is exactly such a product intent, so bot joins SESSION_START_MODE_SPECS and the adapter names it instead of asking for explore. Unlike Deep Research it carries no name of its own — prepareCreate resolves mode?.name ?? input.name, and a fixed spec name would flatten 飞书 任务 / Telegram 任务 into one label — so SessionStartModeSpec.name becomes optional. Its mode:bot label is reserved for free, since prepareCreate already refuses caller-supplied mode labels.

The registry itself moves from deep-research.ts to session-start-mode.ts. It was born there when Deep Research was its only member; bot is a sibling, not a Deep Research detail, and nobody looks for Bot permissions in a file named for Deep Research. Pure move — the importers that only wanted the generic symbols follow the new path.

A mode without a name of its own also exposed a sibling defect: sessions:create dropped the requested name whenever a mode was present, which held only while every mode carried one. It now forwards the name either way and leaves the precedence to the Host. The Bot adapter talks to the Host directly and never took that path, so this is a contract repair, not a second user-visible bug.

Deliberately not covered: bot-incoming-main.ts still swallows invalid_request into 机器人对话处理失败, which is what made this take a packaged-app patch to diagnose. updateSessionConfiguration still admits explore with no mode — the path prepareSession uses to re-arm a bound Bot session.

Before / after, session creation with the input the Feishu Bot actually sends:

BEFORE session.create => {"ok":false,"error":{"code":"invalid_request","message":"Session creation requires a declared mode for explore permission"}}

AFTER  session.create => ok: true persisted: {"name":"飞书 任务","labels":["bot","feishu","mode:bot"],"permissionMode":"explore"}

Both regressions are guarded where they were rejected — at the coordinator and at the IPC handler. The Bot adapter's own test never reached either: its fake client returns a session without entering prepareCreate, which is why this shipped.

Generative tooling: Claude Code (Opus 5) wrote this change; commits carry a Generated-by trailer.