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.spec.ts

@@ -35,7 +35,10 @@ function makeReq(headers: Record<string, any> = {}) {
   return { headers } as any;
 }
 
-const VALID_ID = 'aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee';
+// A syntactically valid v4 UUID (version nibble 4, variant nibble 8). The
+// shared `uuid` validator is stricter than a bare hex-shape regex, so the id
+// must carry a real version/variant.
+const VALID_ID = 'aaaaaaaa-bbbb-4ccc-8ddd-eeeeeeeeeeee';
 
 function entry(buf: Buffer, mime: string, sha256: string): SandboxEntry {
   return { buf, mime, sha256, expiresAt: Date.now() + 60_000 };