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/web/schemas.ts

@@ -70,7 +70,10 @@ const OpenCodeConfigSchema = z.object({
   autoAllowTools: z.boolean().optional(),
   continueSession: z.string().max(100).regex(/^[a-zA-Z0-9_-]+$/).optional(),
   forkSession: z.boolean().optional(),
-  configContent: z.string().max(10000).optional(),
+  configContent: z.string().max(10000).refine(
+    (val) => { try { JSON.parse(val); return true; } catch { return false; } },
+    { message: 'configContent must be valid JSON' },
+  ).optional(),
 }).optional();
 
 export const CreateSessionSchema = z.object({