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>

pnpm-lock.yaml

@@ -63,6 +63,9 @@ importers:
       '@modelcontextprotocol/sdk':
         specifier: ^1.29.0
         version: 1.29.0(zod@3.25.76)
+      '@opencode-ai/sdk':
+        specifier: ~1.15.0
+        version: 1.15.12
       fastify:
         specifier: ^4.28.1
         version: 4.29.1
@@ -920,6 +923,9 @@ packages:
   '@open-draft/until@2.1.0':
     resolution: {integrity: sha512-U69T3ItWHvLwGg5eJ0n3I62nWuE6ilHlmz7zM0npLBRvPRd7e6NYmg54vvRtP5mZG7kZqZCFVdsTWo7BPtBujg==}
 
+  '@opencode-ai/sdk@1.15.12':
+    resolution: {integrity: sha512-lOaBNX93dkakZe6C42ttX1bkSx3K2c6+Yv+w8Qv02v5rPlu1vCXbmdfYDh9/bw+oq+NKPSaBm9d6kPA19hA5Lg==}
+
   '@opentelemetry/api@1.9.1':
     resolution: {integrity: sha512-gLyJlPHPZYdAk1JENA9LeHejZe1Ti77/pTeFm/nMXmQH/HFZlcS/O2XJB+L8fkbrNSqhdtlvjBVjxwUYanNH5Q==}
     engines: {node: '>=8.0.0'}
@@ -4702,6 +4708,10 @@ snapshots:
 
   '@open-draft/until@2.1.0': {}
 
+  '@opencode-ai/sdk@1.15.12':
+    dependencies:
+      cross-spawn: 7.0.6
+
   '@opentelemetry/api@1.9.1': {}
 
   '@pinojs/redact@0.4.0': {}