Let hosted agents discover and message team channels #180

Let an agent in a hosted channel discover and message other team channels through the injected delivery boundary. Hosted channels currently lack outbound delivery. #57 and draft #160 address agents on local peer hosts; they do not complete this hosted path.

Acceptance

• Discover eligible local and hosted destinations with stable channel identity and enough project/owner/host or cell context to select the intended channel. Handle duplicate names explicitly.
• Deliver a human-visible agent message and optional agent invocation from a hosted source to local and hosted destinations; support replies by stable source id after a rename.
• Verify source attribution from the existing team authority, not caller-supplied author data. Enforce destination sharing policy and keep agents in different channels as messaging peers, not parent/child owners.
• Preserve idempotent delivery across lost replies/reconnect/replay and expose clear offline/refused outcomes. Do not introduce accidental multi-hop loops or a second durable channel-message store.
• Verify with deployed hosted channels, real models and a local workspace/host: hosted-to-local, local-to-hosted reply and hosted-to-hosted; include sharing off/on, rename, duplicate delivery and reconnect.

Keep packages/channel runtime-neutral with injected discovery/delivery, pi-durable authoritative, and the tailnet the sole membership/collaboration authority. Define the cell-to-team transport within that model instead of treating a shared service secret as a participant identity.

Priority 5 under #13. Reuse the relevant #57/#160 routing and attribution work where appropriate; completing every peer-host acceptance case is not a substitute for this hosted acceptance.