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/bun/index.ts

@@ -2,6 +2,7 @@ import path from "path";
 import { BrowserWindow, BrowserView, Updater } from "electrobun/bun";
 import { PtyClient } from "./pty-client.ts";
 import { settingsDb } from "./settings-db.ts";
+import { SidecarManager } from "./sidecar-manager.ts";
 import type { PtyRPCSchema } from "../shared/pty-rpc-schema.ts";
 import { randomUUID } from "crypto";
 
@@ -11,6 +12,7 @@ const DEV_SERVER_URL = `http://localhost:${DEV_SERVER_PORT}`;
 // ── PTY daemon client ────────────────────────────────────────────────────────
 
 const ptyClient = new PtyClient();
+const sidecarManager = new SidecarManager();
 
 async function connectToDaemon(retries = 5, delayMs = 500): Promise<boolean> {
   for (let attempt = 1; attempt <= retries; attempt++) {
@@ -347,6 +349,90 @@ const rpc = BrowserView.defineRPC<PtyRPCSchema>({
           return { ok: false };
         }
       },
+
+      // ── Agent handlers ──────────────────────────────────────────────────
+
+      "agent.start": ({ sessionId, provider, prompt, cwd, model, systemPrompt, maxTurns, permissionMode, claudeConfigDir, extraEnv }) => {
+        try {
+          const result = sidecarManager.startSession(sessionId, provider, prompt, {
+            cwd,
+            model,
+            systemPrompt,
+            maxTurns,
+            permissionMode,
+            claudeConfigDir,
+            extraEnv,
+          });
+
+          if (result.ok) {
+            // Forward sidecar messages to webview
+            sidecarManager.onMessage(sessionId, (sid, messages) => {
+              try {
+                rpc.send["agent.message"]({ sessionId: sid, messages });
+              } catch (err) {
+                console.error("[agent.message] forward error:", err);
+              }
+            });
+
+            sidecarManager.onStatus(sessionId, (sid, status, error) => {
+              try {
+                rpc.send["agent.status"]({ sessionId: sid, status, error });
+              } catch (err) {
+                console.error("[agent.status] forward error:", err);
+              }
+
+              // Send cost update on status change
+              const sessions = sidecarManager.listSessions();
+              const session = sessions.find((s) => s.sessionId === sid);
+              if (session) {
+                try {
+                  rpc.send["agent.cost"]({
+                    sessionId: sid,
+                    costUsd: session.costUsd,
+                    inputTokens: session.inputTokens,
+                    outputTokens: session.outputTokens,
+                  });
+                } catch { /* ignore */ }
+              }
+            });
+          }
+
+          return result;
+        } catch (err) {
+          const error = err instanceof Error ? err.message : String(err);
+          console.error("[agent.start]", err);
+          return { ok: false, error };
+        }
+      },
+
+      "agent.stop": ({ sessionId }) => {
+        try {
+          return sidecarManager.stopSession(sessionId);
+        } catch (err) {
+          const error = err instanceof Error ? err.message : String(err);
+          console.error("[agent.stop]", err);
+          return { ok: false, error };
+        }
+      },
+
+      "agent.prompt": ({ sessionId, prompt }) => {
+        try {
+          return sidecarManager.writePrompt(sessionId, prompt);
+        } catch (err) {
+          const error = err instanceof Error ? err.message : String(err);
+          console.error("[agent.prompt]", err);
+          return { ok: false, error };
+        }
+      },
+
+      "agent.list": () => {
+        try {
+          return { sessions: sidecarManager.listSessions() };
+        } catch (err) {
+          console.error("[agent.list]", err);
+          return { sessions: [] };
+        }
+      },
     },
 
     messages: {},