feat(chat): start a session directly as an agent profile

[SoulKyu]

Summary

Agent profiles could only run as subagents, dispatched by the default agent through the Agent tool. There was no way to talk to a specialized agent directly: every conversation started as the default agent. This adds an Agent selector to the new-session composer. Picking a profile starts a top-level session that runs as that profile from its first message.

It reuses what subagent sessions already do rather than adding a second mechanism. startRpcSession already restores a persisted subagent's isolated resources (exact or appended prompt, fixed tools, skill/extension switches) from its resource snapshot; a profile session pins the same snapshot, without a parent.

Server

• POST /api/agent/new accepts agentProfile. The profile is resolved with resolveSubagentProfile() (enabled profiles only, same precedence as Agent); an unknown or disabled name is refused.
• The new session gets the profile's prompt plan (buildSubagentPromptPlan()), tools, model and thinking level. The snapshot is written to a new pi-web:agent-profile custom entry once the profile's extension tools are resolved.
• readSubagentSessionResources() also reads that entry, so reopening the session restores the profile, set_tools is refused ("fixed by its profile"), and navigate_tree keeps the restored loadout, exactly as for subagents. readSubagentRun() ignores it, so the session is never filed under a parent.
• Completion notifications stay on: unlike a subagent, this session has no parent to report to.
• GET /api/sessions/[id] returns agentProfile.
• The extension-tool selection for a profile is extracted into resolveProfileActiveTools(), shared with the subagent runtime (no behavior change there).

Client

• components/AgentProfileSelector.tsx: a searchable dropdown (profile lists can be long), filtering on name, display name and description. It is editable only before the first message; afterwards it shows the session's profile read-only.
• The tool preset control is hidden for a profile session, since the profile fixes its tools.
• The composer may already have started an empty runtime (ensure_session) to load commands. Changing the profile drops it; it was never persisted or listed. The next ensureNewSession then starts one with the chosen profile.
• New i18n keys in en, zh-CN, zh-TW.

Test plan

• tsc --noEmit, npm run lint, npm run build
• npm test: 2285/2285 pass, including new lib/subagents.test.mjs cases. They check that an agent-profile entry restores its resources and is not read as a subagent run, and that a malformed entry is ignored.
• Against a running build on a loopback port:
  • ensure_session with agentProfile → get_state.systemPrompt carries the profile's prompt;
  • set_tools is refused, and an unknown profile is rejected;
  • a prompt is answered in the profile's persona, the session file holds the pi-web:agent-profile entry, and GET /api/sessions/[id] returns agentProfile;
  • after a server restart, reopening the session restores the profile's prompt.
• Used manually in the browser: pick a profile, chat with it, reopen the session.