Fix FreshOpenCode cwd routing with one sidecar

[danshapiro]

Summary

• Replaces cwd-scoped OpenCode serve sidecars with one shared sidecar routed by OpenCode's directory query parameter.
• Keeps FreshOpenCode materialization persistence, global event handling, cwd validation, and routed status/history/control requests.
• Emits lost-session errors when the shared sidecar exits or is discarded, so pending waits do not hang.

Verification

• npm run test:vitest -- --config vitest.server.config.ts test/unit/server/fresh-agent/opencode-serve-manager.test.ts test/unit/server/fresh-agent/opencode-serve-events.test.ts test/unit/server/fresh-agent/opencode-serve-adapter.test.ts --run
• npm run test:opencode-serve-smoke
• FRESHELL_TEST_SUMMARY="opencode single sidecar routing rebased" npm run check