feat(sandbox): Phase 7 PR-A — @anthropic-ai/sandbox-runtime dep + doctor + ADR 0014

Lays the foundation for multi-tenant provider spawning isolation per
ADR 0014 § Decision. NO production wiring — PR-B (anthropic.mjs spawn
wrap) lands separately and requires bubblewrap + socat + ripgrep
installed on PI231 first.

Files:
- package.json: add @anthropic-ai/sandbox-runtime ^0.0.52
- lib/sandbox/doctor.mjs (new): preflight checkSandboxAvailability +
  describeSandboxStatus; pure module, no state, no initialize() call
- server.mjs: /health response gains 'sandbox' field with availability
  + missing deps + install hint; result memoized via _sandboxStatusCache;
  __resetSandboxStatusCache() test seam exported
- docs/adr/0014-sandbox-runtime-integration.md (new): 4-PR layered
  rollout decision per Iron Rule 11; PR-B/C/D acceptance criteria
  previewed; 6 spike pitfalls recorded; 282 lines
- CHANGELOG.md: Unreleased entry under Phase 7 PR-A
- test-features.mjs Suite 42 (+8 tests, 797 → 805, all pass)

Authority:
- @anthropic-ai/sandbox-runtime v0.0.52 — anthropic-experimental org
  https://github.com/anthropic-experimental/sandbox-runtime
- 2026-05-28 PoC spike (verdict YELLOW; report at /tmp/sandbox-spike/
  on PI231): clean arm64 install, dep check fail-closed behaviour
  confirmed, three PoC scripts parked
- cc-mem incident memory § 4 (prior-art search — ecosystem hasn't
  solved multi-tenant fs/tool isolation)
- ADR 0009 Amendment 1 § Caveats #3 — sandbox is cloud prerequisite
- docs/plans/cloud-deployment-family.md § 5

Spike note (macOS): on dev Mac mini with rg via Homebrew,
SandboxManager.isSupportedPlatform()=true and checkDependencies()
returns no errors — macOS uses built-in sandbox-exec, not bwrap.
/health.sandbox.available=true on macOS dev, false on PI231 until
apt install.

Operational follow-ups (NOT this PR):
1. sudo apt-get install -y bubblewrap socat ripgrep on PI231 (5-min window)
2. PR-B: lib/providers/anthropic.mjs spawn wrap + negative test
   (in-sandbox cat of OAuth token MUST fail)
3. PR-C: lib/providers/codex.mjs wrap with enableWeakerNestedSandbox:true
4. PR-D: cloud deployment plan update + unblock

Tests: 797 → 805 (all pass, 0 fail).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

package.json

@@ -49,5 +49,8 @@
     "mistral",
     "fallback",
     "cache"
-  ]
+  ],
+  "dependencies": {
+    "@anthropic-ai/sandbox-runtime": "^0.0.52"
+  }
 }