fix: address code review findings for OpenCode integration

- Add _isStopped guard to OpenCode 3s readiness timeout (session.ts)
- Block respawn for opencode sessions on interactive-respawn and
  respawn/enable routes (server.ts)
- Fail fast in direct PTY fallback for OpenCode mode (session.ts)
- Validate configContent as JSON at schema level (schemas.ts)
- Update JSDoc example for createSession options API (tmux-manager.ts)
- Un-hide Context tab for OpenCode sessions (index.html)
- Add OpenCode UI tests (opencode-resize.test.ts)

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

src/tmux-manager.ts

@@ -258,7 +258,7 @@ function setOpenCodeConfigContent(muxName: string, config?: OpenCodeConfig): voi
  * const manager = new TmuxManager();
  *
  * // Create a tmux session for Claude
- * const session = await manager.createSession(sessionId, '/project', 'claude');
+ * const session = await manager.createSession({ sessionId, workingDir: '/project', mode: 'claude' });
  *
  * // Send input (single command, no delay!)
  * manager.sendInput(sessionId, '/clear\r');