feat(orchestration): multi-agent communication, unified agents, env passthrough

- btmsg: admin role (tier 0), channel messaging (create/list/send/history),
  admin global feed, mark-read conversations
- Rust btmsg module: admin bypass, channels, feed, 8 new Tauri commands
- CommsTab: sidebar chat interface with activity feed, DMs, channels (Ctrl+M)
- Agent unification: Tier 1 agents rendered as ProjectBoxes via agentToProject()
  converter, getAllWorkItems() combines agents + projects in ProjectGrid
- GroupAgentsPanel: click-to-navigate agents to their ProjectBox
- Agent system prompts: generateAgentPrompt() builds comprehensive introductory
  context (role, environment, team, btmsg/bttask docs, workflow instructions)
- AgentSession passes group context to prompt generator via $derived.by()
- BTMSG_AGENT_ID env var passthrough: extra_env field flows through full chain
  (agent-bridge → Rust AgentQueryOptions → NDJSON → sidecar runners → cleanEnv)
- workspace store: updateAgent() for Tier 1 agent config persistence

v2/src/lib/adapters/agent-bridge.ts

@@ -23,6 +23,8 @@ export interface AgentQueryOptions {
   /** When set, agent runs in a git worktree for isolation */
   worktree_name?: string;
   provider_config?: Record<string, unknown>;
+  /** Extra environment variables injected into the agent process (e.g. BTMSG_AGENT_ID) */
+  extra_env?: Record<string, string>;
   remote_machine_id?: string;
 }