fix(web): agents are addressed only by run address (CL-8567)

[TheGreatAxios]

Summary

• Myra guessed a slug-based mail address (scribe@<domain>) for a handoff instead of using the recipient's run address, and the hub dropped the message.

Changes

• Added why-comments at the mail trigger construction sites (apps/web/src/agent-deploy.ts, apps/web/src/myra-deploy.ts) clarifying the trigger's to value only feeds deploy-time mail grants and is not a routable address.
• Added a short paragraph to Myra's system prompt stating an agent's real address is its run address, found only in the Participants: block of incoming mail, and that an address must never be constructed from an agent's name/slug.
• Audited apps/web/src for any UI surface printing a slug/trigger address as an agent's address; found none — the Agents page, room roster, and participants panel already resolve addresses from the live run/deployment listing (listRoomParticipants, chatAgents.liveAddress), so no UI changes were needed.

Testing

• bun test apps/web/src/agent-deploy.test.ts apps/web/src/myra-deploy.test.ts agents/myra/src
• bun run check (typecheck, lint, fmt, test) — all green

[linear-code]

CL-8567