feat(P01): sandbox backend + workdir + deps + config (Wave 1)

Task 1-1-01: SandboxBackend protocol + UnshareBackend (unshare user/mount/pid/net)
+ workdir layout (workspace/ + snapshots/). Isolation probe real on this box:
in-ns uid=0, network isolated (fresh net ns, lo only), writes contained to per-sandbox
bind dir; proc-remount not permitted (documented, not required). Deviations (documented):
util-linux 2.38.1 lacks --bind flag -> bind moved into namespace via sh -c mount shim;
rlimits moved into shim (preexec would kill the pytest interpreter); userns != DAC barrier
(host-uid-owned targets), documented for hardening under D-025.

Task 1-1-02: add sqlmodel, sqlalchemy, websockets, aiofiles (PyPI-verified); config keys
SANDBOX_DIR/MAX_CONCURRENT=5/TIMEOUT_S=900/MAX_WORKDIR_MB=512/DB_PATH (D-024/027/032/G-2).

Task 1-1-03: README sandbox isolation section with real probe transcript + locked
resource-limit mechanism (G-1/G-2); ruff config already correct (no change).

139/139 tests pass; ruff clean. .gitignore: ignore all ai-service sandboxes dirs (runtime+tests).

---ci---
phase: 1
milestone: v0.3
status: execute
requirements: {covered: [REQ-3-001, REQ-3-002], partial: []}
---/ci---

.gitignore

@@ -49,3 +49,9 @@ coverage/
 .ruff_cache/
 *.egg-info/
 .ciagent/bin/
+
+# v0.3 engine runtime data (SQLite + sandboxes)
+apps/ai-service/ai_service/data/
+apps/ai-service/**/sandboxes/
+*.db
+*.db-journal