[sandbox-ext] Phase 3: parameter surface — schema + project field + verb override + cache-by-features

Migration 072 adds projects.sandbox_extensions (jsonb null): a per-service
extension/module map a venture declares up front (e.g. {"postgres":
["vector","postgis"],"redis":["search"]}). Additive + nullable so
existing opted-in projects stay byte-for-byte bare — no default set, opters
set the extensions they need explicitly (TimescaleDB out unless asked).

Project model validates the map against SANDBOX_ENGINE_FEATURES: unknown
service keys and unallowed features are rejected at the model boundary with
the allowlist named (plpython3u — superuser-RCE — excluded by construction),
empty feature lists drop to bare, order normalized + deduped. The allowlist
is the security containment, not privilege. Mirrors sandbox_services: not on
ProjectCreate, only Project + ProjectUpdate.

request_sandbox gains an extensions arg; _sandbox_features_scope unions a
per-call override with the project's standing set (trusted), bounds it to the
opted set + allowlist, rejects a non-opted service or unallowed feature with
the allowlist named in remediate — scope-first priority preserved by
rej_scope or rej_features. ensure_sandbox threads features through to
provision(); cache-by-features: a cached entry satisfies a new call iff
services are a subset AND every requested feature per service is already
cached — a feature superset re-provisions (rotates creds), mirroring the
services-superset case. available_extensions rides the evidence payload so an
agent doesn't guess what was activated.

Gate: ruff clean, mypy clean (9 modules), 51 tests pass (incl. migration
round-trip).

tests/integration/test_migration_sandbox_extensions.py

@@ -0,0 +1,77 @@
+"""Per-project sandbox extensions opt-in column (migration 072).
+
+Migration 072 adds ``projects.sandbox_extensions`` (jsonb null). The real
+upgrade/downgrade chain is verified separately against a throwaway Postgres;
+these assertions guard the resulting schema shape and a value round-trip.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+from uuid import uuid4
+
+import pytest
+from roboco.db.tables import AgentTable, ProjectTable
+from roboco.models import AgentRole, AgentStatus, Team
+from sqlalchemy import select
+
+if TYPE_CHECKING:
+    from sqlalchemy.ext.asyncio import AsyncSession
+
+
+async def _seed_project(db_session: AsyncSession) -> ProjectTable:
+    agent = AgentTable(
+        id=uuid4(),
+        name="Dev",
+        slug=f"be-dev-{uuid4().hex[:8]}",
+        role=AgentRole.DEVELOPER,
+        team=Team.BACKEND,
+        status=AgentStatus.ACTIVE,
+        model_config={},
+        system_prompt="dev",
+        capabilities=[],
+        permissions={},
+        metrics={},
+    )
+    db_session.add(agent)
+    await db_session.flush()
+    project = ProjectTable(
+        id=uuid4(),
+        name="B-Proj",
+        slug=f"b-proj-{uuid4().hex[:8]}",
+        git_url="https://example.com/r.git",
+        assigned_cell=Team.BACKEND,
+        created_by=agent.id,
+    )
+    db_session.add(project)
+    await db_session.flush()
+    return project
+
+
+@pytest.mark.asyncio
+async def test_sandbox_extensions_column_default_null(
+    db_session: AsyncSession,
+) -> None:
+    project = await _seed_project(db_session)
+    assert project.sandbox_extensions is None
+
+
+@pytest.mark.asyncio
+async def test_sandbox_extensions_column_round_trip(
+    db_session: AsyncSession,
+) -> None:
+    project = await _seed_project(db_session)
+    project.sandbox_extensions = {
+        "postgres": ["vector", "postgis"],
+        "redis": ["search"],
+    }
+    await db_session.flush()
+    row = (
+        await db_session.execute(
+            select(ProjectTable).where(ProjectTable.id == project.id)
+        )
+    ).scalar_one()
+    assert row.sandbox_extensions == {
+        "postgres": ["vector", "postgis"],
+        "redis": ["search"],
+    }