feat(web): agents copy the person on handoff mail (CL-8577)

[TheGreatAxios]

Summary

• Agent-to-agent mail in a workbench goes run to run and never lands in the person's mailbox, so a handoff was invisible in the transcript.
• The roster block every room send appends now lists the person alongside the agents and tells agents to copy that address on any handoff.

Changes

• apps/web/src/chat/room-roster.ts: RosterEntry gains a kind ("person" | "agent"); appendRoster adds a trailing instruction naming every person in the roster and telling agents to copy them in to on a handoff. stripRoster needed no change — it already discards everything from the Participants: marker onward.
• apps/web/src/chat/threads-api.ts: sendToRoom now takes the full participant list (not just agents), builds the roster from the person(s) plus live agents, while to still targets only live agents.
• apps/web/src/workbench-create.ts / apps/web/src/pages/workbench-room-page.tsx: pass the full participant list through to sendToRoom.
• agents/myra/src/system-prompt.ts: one added sentence stating the same rule.

Testing

• bun test apps/web/src/chat/room-roster.test.ts
• bun run check (typecheck, lint, fmt, full test suite) — green

Findings on mail_send cc/comma support

• @intx/tools-mail (0.3.0) mail_send schema has no cc field — only to: string | string[].
• The underlying createOutboundMessage builder (vendor/intx/mime/src/mail-builder.ts) does support a cc field, but the mail tool never exposes it.
• A comma-separated string in to is not viable: each string is validated as a single RFC-5322-ish address with /^[^@\s]+@[^@\s]+$/, so "a@x,b@x" fails validation. Multiple recipients must be passed as an array.
• The instruction added therefore tells agents to add the person's address as another to recipient, not a cc.

Reader check

• readRoomFolder/readRoom in threads-api.ts do not filter messages by participant/address match (unlike the one-on-one chat reader) — every message in the room mailbox's INBOX/Sent folders is already shown, author included, regardless of whether the person is a primary or secondary to recipient. No reader change was needed.

[linear-code]

CL-8577