v0.2.0 — always-on session briefing

Every session now hears about the channel, not just sessions that already have a peer.

• A lone session gets a compact declaration nudge (name, role, goal, summary), so it is addressable when another session appears.
• The full coordination briefing takes over as soon as a peer or an active lease exists.
• announce: false remains the only way to remove the briefing.

Also seeded .opencode/wiki/ with the runtime, install, and communication-wall notes.