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/ralph-wizard.js

@@ -151,11 +151,11 @@ Object.assign(CodemanApp.prototype, {
       const res = await fetch(`/api/cases/${encodeURIComponent(caseName)}/fix-plan`);
       const data = await res.json();
 
-      if (data.success && data.exists && data.todos?.length > 0) {
+      if (data.success && data.data.exists && data.data.todos?.length > 0) {
         this.ralphWizardConfig.existingPlan = {
-          todos: data.todos,
-          stats: data.stats,
-          content: data.content,
+          todos: data.data.todos,
+          stats: data.data.stats,
+          content: data.data.content,
         };
         this.updateExistingPlanUI();
       } else {
@@ -1054,8 +1054,8 @@ Object.assign(CodemanApp.prototype, {
         this.showToast(data.error || 'Failed to start', 'error');
         return;
       }
-      this.ralphClosedSessions.delete(data.sessionId);
-      await this.selectSession(data.sessionId);
+      this.ralphClosedSessions.delete(data.data.sessionId);
+      await this.selectSession(data.data.sessionId);
       this.showToast(`Ralph Loop started in ${config.caseName}`, 'success');
     } catch (err) {
       console.error('Failed to start Ralph loop:', err);