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>

src/web/public/panels-ui.js

@@ -251,7 +251,7 @@ Object.assign(CodemanApp.prototype, {
       const response = await fetch('/api/token-stats');
       const data = await response.json();
       if (data.success) {
-        this.renderTokenStats(data);
+        this.renderTokenStats(data.data);
         document.getElementById('tokenStatsModal').classList.add('active');
       } else {
         this.showToast('Failed to load token stats', 'error');
@@ -2881,7 +2881,7 @@ Object.assign(CodemanApp.prototype, {
     try {
       const res = await fetch('/api/mux-sessions');
       const data = await res.json();
-      this.muxSessions = data.sessions || [];
+      this.muxSessions = data.data?.sessions || [];
       this.renderMuxSessions();
     } catch (err) {
       console.error('Failed to load mux sessions:', err);
@@ -3109,8 +3109,8 @@ Object.assign(CodemanApp.prototype, {
       const res = await fetch('/api/mux-sessions/reconcile', { method: 'POST' });
       const data = await res.json();
 
-      if (data.dead && data.dead.length > 0) {
-        this.showToast(`Found ${data.dead.length} dead mux session(s)`, 'warning');
+      if (data.data?.dead && data.data.dead.length > 0) {
+        this.showToast(`Found ${data.data.dead.length} dead mux session(s)`, 'warning');
         await this.loadMuxSessions();
       } else {
         this.showToast('All mux sessions are alive', 'success');
@@ -3220,7 +3220,7 @@ Object.assign(CodemanApp.prototype, {
     try {
       const res = await fetch('/api/system/stats');
       const stats = await res.json();
-      this.updateSystemStatsDisplay(stats);
+      this.updateSystemStatsDisplay(stats.data);
     } catch (err) {
       // Silently fail - system stats are not critical
     }