fix(web): workbench mail asks agents to give every mail a subject (CL-8584)

[TheGreatAxios]

Summary

• The roster instruction on every workbench message and Myra's handoff sentence ask agents to give every mail a short subject
• A mail with an empty Subject ends the receiving agent's run today; this keeps handoffs alive until the platform tolerates it

Changes

• apps/web/src/chat/room-roster.ts and its test: instruction line
• agents/myra/src/system-prompt.ts: handoff sentence

Testing

• Roster unit tests pass; prettier clean

[linear-code]

CL-8584