refactor(sandbox): address PR #250 round-3 review — dead import, env validation, uuid validator, docs (#243)

Must-fix:
- mcp.module: drop the now-dead EnvironmentModule import (and its stale
  comment). McpService no longer injects EnvironmentService; EnvironmentModule
  is @Global and imported at the app root, so DI still resolves.

Stability:
- environment.service: route getSandboxTtlMs + the three SANDBOX_MAX_*_BYTES
  caps through a shared getPositiveIntEnv() helper that warns once per key and
  falls back to the default on a non-integer or <= 0 value (previously the byte
  caps did a bare parseInt, so SANDBOX_MAX_TOTAL_BYTES=0 made every stash_page
  fail against a 0-byte cap). TTL behavior is unchanged.

Simplification:
- sandbox.controller: replace the homemade UUID_RE with the project's shared
  `uuid` validator (import { validate as isValidUUID } from 'uuid'), matching
  the attachment routes; update the spec fixtures to valid v4 UUIDs.
- mcp.service: inline the single-caller one-liner buildSandboxConfig() to
  this.sandboxStore.asSink() at the wiring site.

Docs:
- CHANGELOG: add an [Unreleased] > Added entry for #243 (stash_page tool,
  anonymous GET /api/sb/:id, five SANDBOX_* env vars).
- AGENTS.md: note that GET /api/sb/:id is in the workspace-gate preHandler's
  excludedPaths and is fully tokenless, unlike /api/files/public/... which
  still resolves a workspace and needs an attachment JWT.

Tests: cap-getter validation (0/-5/abc -> default, valid -> parsed), updated
UUID fixtures. apps/server jest sandbox/environment/mcp: 233 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/integrations/sandbox/sandbox.controller.ts

@@ -1,14 +1,9 @@
 import { Controller, Get, Param, Req, Res } from '@nestjs/common';
 import { FastifyReply, FastifyRequest } from 'fastify';
+import { validate as isValidUUID } from 'uuid';
 import { SandboxStore } from './sandbox.store';
 import { SANDBOX_ROUTE_SEGMENT } from './sandbox.constants';
 
-// Strict UUID v-agnostic shape. This is anti-traversal / input hygiene (so `:id`
-// can never be a path like `../...`), NOT authorization — the capability is the
-// unguessable id itself plus the short TTL plus TLS.
-const UUID_RE =
-  /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/;
-
 // MIME types safe to render inline in a browser. SVG is deliberately EXCLUDED
 // (it can carry script), as are text/html and the JSON document blob — anything
 // not on this list is served as an attachment so an attacker-controlled mime can
@@ -51,9 +46,13 @@ export class SandboxController {
     @Req() req: FastifyRequest,
     @Res() res: FastifyReply,
   ): Promise<void> {
-    // Non-UUID id (including any traversal attempt) → 404 before touching the
-    // store. No stack trace leaks out.
-    if (!UUID_RE.test(id)) {
+    // Validate `:id` as a real UUID via the shared `uuid` validator (same as the
+    // attachment routes). This is anti-traversal / input hygiene (so `:id` can
+    // never be a path like `../...`), NOT authorization — the capability is the
+    // unguessable id itself plus the short TTL plus TLS. A non-UUID id (including
+    // any traversal attempt) → 404 before touching the store; no stack trace
+    // leaks out.
+    if (!isValidUUID(id)) {
       res.status(404).send();
       return;
     }