BO-7 phase 2: containerize the App Workshop app-builder's session

[vjvarada]

Summary

Follow-up to #255 (code_task containerization): wires the App Workshop app-builder's interactive chat session into the same copilot_sandbox.py sandbox — the second, previously-unwired call site it was already built to cover.

• New orchestrator.executor._maybe_sandbox_session_workspace(), gated on the same allow_session_workspace hook _session_workspace_override already uses to identify this agent shape (never a hardcoded agent name), plus "app_builder" in copilot_sandbox_scope (still default-off — zero behavior change until explicitly enabled).
• Uses copilot_sandbox.get_or_spawn_sticky's per-thread_id container reuse: unlike code_task's one-shot spawn+stop, a fresh container on every chat turn would regress the latency this interactive path doesn't have today.
• Extracted as its own function rather than left inline, so it's directly unit-testable — matching how _session_workspace_override/_resolve_effective_agent_dir are already factored out nearby.

The T2 build step needed real accommodation, not just a mount. build_t2.mjs runs via node, and its host-absolute script path is baked into the app-builder's own system prompt at import time. Mounting the app-builder's repo clone at the identical container path makes that baked-in path resolve correctly with zero prompt/agent changes. The shared T2 vendor cache (react/react-dom/esbuild) mounts at a fresh container path, pointed to via CUSTOM_APPS_T2_VENDOR_DIR — an env var build_t2.mjs already reads on the host, so no script changes either. copilot_sandbox.py gained extra_env support (non-secret only) for this, and Dockerfile.copilot-sandbox picked up nodejs.

Test plan

• tests/unit/test_app_builder_sandbox.py — gating (session-workspace agent + scope, both required), mount/env composition, fall-back-on-spawn-failure
• docker compose --profile sandbox config validates the updated image
• pytest tests/unit/ — 2313 passed, 0 regressions

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