Accept story 30: Worktree-based agent orchestration

Add git worktree isolation for concurrent story agents. Each agent now
runs in its own worktree with setup/teardown commands driven by
.story_kit/project.toml config. Agents stream output via SSE and support
start/stop lifecycle with Pending/Running/Completed/Failed statuses.

Backend: config.rs (TOML parsing), worktree.rs (git worktree lifecycle),
refactored agents.rs (broadcast streaming), agents_sse.rs (SSE endpoint).
Frontend: AgentPanel.tsx with Run/Stop buttons and streaming output log.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/Cargo.toml

@@ -22,6 +22,9 @@ rust-embed = { workspace = true }
 mime_guess = { workspace = true }
 homedir = { workspace = true }
 serde_yaml = "0.9"
+toml = "0.8"
+async-stream = "0.3"
+bytes = "1"
 portable-pty = { workspace = true }
 strip-ansi-escapes = { workspace = true }