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/sse-subscription-filter.test.ts

@@ -159,7 +159,7 @@ describe('SSE Subscription Filtering', () => {
       body: JSON.stringify({ workingDir: '/tmp' }),
     });
     const createData = await createRes.json();
-    const sessionId = createData.session.id;
+    const sessionId = createData.data.session.id;
 
     // Wait for events to arrive
     await new Promise((resolve) => setTimeout(resolve, 300));
@@ -210,7 +210,7 @@ describe('SSE Subscription Filtering', () => {
       body: JSON.stringify({ workingDir: '/tmp' }),
     });
     const createData = await createRes.json();
-    const sessionId = createData.session.id;
+    const sessionId = createData.data.session.id;
 
     // Now connect SSE with the session filter
     const controller = new AbortController();
@@ -264,14 +264,14 @@ describe('SSE Subscription Filtering', () => {
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ workingDir: '/tmp' }),
     });
-    const session1 = (await createRes1.json()).session;
+    const session1 = (await createRes1.json()).data.session;
 
     const createRes2 = await fetch(`${baseUrl}/api/sessions`, {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ workingDir: '/tmp' }),
     });
-    const session2 = (await createRes2.json()).session;
+    const session2 = (await createRes2.json()).data.session;
 
     // Connect SSE subscribed ONLY to session1
     const controller = new AbortController();
@@ -331,14 +331,14 @@ describe('SSE Subscription Filtering', () => {
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ workingDir: '/tmp' }),
     });
-    const session1 = (await createRes1.json()).session;
+    const session1 = (await createRes1.json()).data.session;
 
     const createRes2 = await fetch(`${baseUrl}/api/sessions`, {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ workingDir: '/tmp' }),
     });
-    const session2 = (await createRes2.json()).session;
+    const session2 = (await createRes2.json()).data.session;
 
     // Connect SSE subscribed to both sessions
     const controller = new AbortController();