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/bterminal-core/src/sidecar.rs

@@ -31,6 +31,9 @@ pub struct AgentQueryOptions {
     /// Provider-specific configuration blob (passed through to sidecar as-is)
     #[serde(default)]
     pub provider_config: serde_json::Value,
+    /// Extra environment variables injected into the agent process (e.g. BTMSG_AGENT_ID)
+    #[serde(default)]
+    pub extra_env: std::collections::HashMap<String, String>,
 }
 
 fn default_provider() -> String {
@@ -220,6 +223,7 @@ impl SidecarManager {
             "additionalDirectories": options.additional_directories,
             "worktreeName": options.worktree_name,
             "providerConfig": options.provider_config,
+            "extraEnv": options.extra_env,
         });
 
         self.send_message(&msg)