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/mainview/App.svelte

@@ -383,10 +383,6 @@
             name={project.name}
             cwd={project.cwd}
             accent={project.accent}
-            status={project.status}
-            costUsd={project.costUsd}
-            tokens={project.tokens}
-            messages={project.messages}
             provider={project.provider}
             profile={project.profile}
             model={project.model}