feat(api): establish stable HTTP contract — uniform {success,data} envelope, status codes, /api/v1

Point 1 of the v1.0 lock-in: commit to a stable HTTP API (the cleanest, fullest form).

Core (centralized):
- Every JSON /api response now uses ONE envelope via a Fastify preSerialization hook (src/web/server.ts): success -> { success:true, data:<payload> }; error -> { success:false, error, errorCode } with a conventional HTTP status. Non-JSON routes (file-raw, tail-file SSE, download, screenshots, /q redirect, WS) are skipped.
- Error-code -> HTTP status is a single source of truth (httpStatusForErrorCode in src/types/api.ts): 400/401/404/409/422/429/500. Expanded ApiErrorCode (added UNAUTHORIZED, CONFLICT, RATE_LIMITED). Errors are no longer HTTP 200.
- Versioned alias: /api/v1/* rewrites to /api/* (rewriteApiV1Url), so external clients pin to a stable surface while the bundled UI keeps using /api/*.
- Handlers stripped of manual 'success:true' (50 across 14 route files) so they return bare payloads the hook wraps uniformly; fixed the mux DELETE {success:<bool>} envelope collision (-> {killed}).

Frontend (48 call sites across 10 files):
- _apiJson() auto-unwraps { success:true, data } -> data (null on error), so most bare-shape readers are transparent. Raw-fetch sites relocate payload reads under .data; success/res.ok/error checks unchanged.

Docs: new docs/api-reference.md (envelope, status table, error codes, /api/v1, SSE); versioning-policy.md flipped — the HTTP/SSE API is now part of the stable, SemVer-covered surface.

Verification: full unit/route suite green (2680 passed) incl. ~166 updated assertions across 24 test files; typecheck/lint/format/frontend-syntax clean; a headless-chromium smoke loaded the migrated UI and drove the panels with 0 console/page errors; /api/status and /api/v1/status confirmed returning the uniform envelope live.

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

test/session.test.ts

@@ -32,9 +32,9 @@ describe('Interactive Session Lifecycle', () => {
       const data = await response.json();
 
       expect(data.success).toBe(true);
-      expect(data.session.id).toBeDefined();
-      expect(data.session.workingDir).toBeDefined();
-      expect(data.session.status).toBe('idle');
+      expect(data.data.session.id).toBeDefined();
+      expect(data.data.session.workingDir).toBeDefined();
+      expect(data.data.session.status).toBe('idle');
     });
 
     it('should create session with custom working directory', async () => {
@@ -47,7 +47,7 @@ describe('Interactive Session Lifecycle', () => {
       const data = await response.json();
 
       expect(data.success).toBe(true);
-      expect(data.session.workingDir).toBe('/tmp/test');
+      expect(data.data.session.workingDir).toBe('/tmp/test');
     });
   });
 
@@ -60,14 +60,14 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       // Get the session
       const response = await fetch(`${baseUrl}/api/sessions/${sessionId}`);
       const data = await response.json();
 
-      expect(data.id).toBe(sessionId);
-      expect(data.status).toBeDefined();
+      expect(data.data.id).toBe(sessionId);
+      expect(data.data.status).toBeDefined();
     });
 
     it('should return error for non-existent session', async () => {
@@ -87,7 +87,7 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       // Delete the session
       const response = await fetch(`${baseUrl}/api/sessions/${sessionId}`, {
@@ -123,7 +123,7 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       // Get output
       const response = await fetch(`${baseUrl}/api/sessions/${sessionId}/output`);
@@ -145,14 +145,14 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       // Get terminal buffer
       const response = await fetch(`${baseUrl}/api/sessions/${sessionId}/terminal`);
       const data = await response.json();
 
-      expect(data).toHaveProperty('terminalBuffer');
-      expect(data).toHaveProperty('status');
+      expect(data.data).toHaveProperty('terminalBuffer');
+      expect(data.data).toHaveProperty('status');
     });
   });
 
@@ -165,7 +165,7 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       // Start interactive mode
       const response = await fetch(`${baseUrl}/api/sessions/${sessionId}/interactive`, {
@@ -195,7 +195,7 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       await fetch(`${baseUrl}/api/sessions/${sessionId}/interactive`, {
         method: 'POST',
@@ -225,7 +225,7 @@ describe('Interactive Session Lifecycle', () => {
         body: JSON.stringify({}),
       });
       const createData = await createRes.json();
-      const sessionId = createData.session.id;
+      const sessionId = createData.data.session.id;
 
       await fetch(`${baseUrl}/api/sessions/${sessionId}/interactive`, {
         method: 'POST',