feat(coder): v2.6 Phase 1 — OpenCode warm server backend

Persistent multi-turn opencode backend: one `opencode serve` HTTP server per
BooCoder process, one opencode session per BooCode session (resumed on
switch-back), single SSE read loop demuxed by session id.

- backends/opencode-server.ts: AgentBackend implementation — spawn with
  waitForReady, session.next.* SSE event translation (text/reasoning/tool
  deltas), Paseo-ported reasoning dedup (streamedPartKeys), promptAsync
  fire-and-forget settled by session.idle, per-turn inactivity watchdog
  (180s) + reconnect reconciliation via session.messages, stale-session
  guard (crashed-not-resumed + config_hash fingerprint on model).
- dispatcher.ts: opencode routes to pool backend (ensureSession→prompt);
  per-session concurrency Map replaces global running boolean (1.9);
  model coalesce (empty→DEFAULT_MODEL) + llama-swap/ prefix for opencode;
  diff-supersede (DELETE+INSERT pending_changes by session, stamp agent).
- worktrees.ts: ensureSessionWorktree (session-keyed, captures base_commit,
  persists to session_worktrees); diffWorktree gains optional baseRef.
- agent-probe.ts: mergeLlamaSwap branch fetches /v1/models, prefixes with
  llama-swap/, populates opencode's available_agents.models (was 0).
- provider-snapshot.ts: export fetchLlamaSwapModels for probe reuse.
- schema.sql: session_worktrees + agent_sessions tables (Phase 0) +
  config_hash column on agent_sessions, pending_changes.agent column.
- package.json: @opencode-ai/sdk ~1.15.0 (resolved 1.15.12).

Known Phase 1 limitation: single SSE stream scoped to most-recent session's
directory; concurrent opencode sessions in different worktrees collide
(warning logged, watchdog prevents hang). Phase 2 moves to per-session SSE.

Smoke 1 verified: two turns in one session, both produce real tokens, same
agent_session_id reused, same server port, turn 2 is 9x faster (no spawn).
goose/qwen/claude paths untouched (runExternalAgent md5 identical).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/package.json

@@ -16,6 +16,7 @@
     "@agentclientprotocol/sdk": "^0.22.1",
     "@boocode/server": "workspace:*",
     "@fastify/static": "^7.0.4",
+    "@opencode-ai/sdk": "~1.15.0",
     "@fastify/websocket": "^10.0.1",
     "@modelcontextprotocol/sdk": "^1.29.0",
     "fastify": "^4.28.1",