feat(electrobun): agent execution layer — sidecar manager + message adapters + store

- SidecarManager: spawns claude/codex/ollama runners via Bun.spawn(),
  NDJSON stdio protocol, Claude CLI auto-detection, env stripping,
  AbortController stop, Deno/Node runtime detection
- MessageAdapter: parses Claude stream-json, Codex ThreadEvent, Ollama
  chunks into common AgentMessage format
- agent-store.svelte.ts: per-project reactive session state, RPC event
  listeners for agent.message/status/cost
- AgentPane: wired to real sessions (start/stop/prompt), stop button,
  thinking/system message rendering
- ProjectCard: status dot from real agent status, cost/tokens from store
- 5 new RPC types (agent.start/stop/prompt/list + events)

ui-electrobun/src/shared/pty-rpc-schema.ts

@@ -146,6 +146,50 @@ export type PtyRPCRequests = {
     params: { id: string };
     response: { ok: boolean };
   };
+
+  // ── Agent RPC ─────────────────────────────────────────────────────────────
+
+  /** Start an agent session with a given provider. */
+  "agent.start": {
+    params: {
+      sessionId: string;
+      provider: "claude" | "codex" | "ollama";
+      prompt: string;
+      cwd?: string;
+      model?: string;
+      systemPrompt?: string;
+      maxTurns?: number;
+      permissionMode?: string;
+      claudeConfigDir?: string;
+      extraEnv?: Record<string, string>;
+    };
+    response: { ok: boolean; error?: string };
+  };
+  /** Stop a running agent session. */
+  "agent.stop": {
+    params: { sessionId: string };
+    response: { ok: boolean; error?: string };
+  };
+  /** Send a follow-up prompt to a running agent session. */
+  "agent.prompt": {
+    params: { sessionId: string; prompt: string };
+    response: { ok: boolean; error?: string };
+  };
+  /** List all active agent sessions with their state. */
+  "agent.list": {
+    params: Record<string, never>;
+    response: {
+      sessions: Array<{
+        sessionId: string;
+        provider: string;
+        status: string;
+        costUsd: number;
+        inputTokens: number;
+        outputTokens: number;
+        startedAt: number;
+      }>;
+    };
+  };
 };
 
 // ── Messages (Bun → WebView, fire-and-forget) ────────────────────────────────
@@ -155,6 +199,33 @@ export type PtyRPCMessages = {
   "pty.output": { sessionId: string; data: string };
   /** PTY session exited. */
   "pty.closed": { sessionId: string; exitCode: number | null };
+
+  // ── Agent events (Bun → WebView) ─────────────────────────────────────────
+
+  /** Agent message(s) parsed from sidecar NDJSON. */
+  "agent.message": {
+    sessionId: string;
+    messages: Array<{
+      id: string;
+      type: string;
+      parentId?: string;
+      content: unknown;
+      timestamp: number;
+    }>;
+  };
+  /** Agent session status change. */
+  "agent.status": {
+    sessionId: string;
+    status: string;
+    error?: string;
+  };
+  /** Agent cost/token update. */
+  "agent.cost": {
+    sessionId: string;
+    costUsd: number;
+    inputTokens: number;
+    outputTokens: number;
+  };
 };
 
 // ── Combined schema ───────────────────────────────────────────────────────────