feat(tool): implement BuiltinAdapter class and MCP automatic resource slot creation (#964)

## Summary

Implement `BuiltinAdapter` class and MCP automatic resource slot creation. Two main changes:

### 1. BuiltinAdapter Class (`tool/builtins/adapter.py`)
Formal adapter implementing the tool adapter lifecycle pattern for built-in tools:
- `discover()` — returns all built-in tool descriptors (file, git, subplan tools)
- `register(registry)` — registers all tools in a ToolRegistry, returns registered names
- `activate()`/`deactivate()` — no-ops (built-in tools are always available)
- Wraps existing `ALL_FILE_TOOLS`, `ALL_GIT_TOOLS`, `ALL_SUBPLAN_TOOLS` lists
- Backward compatible — existing registration functions still work

### 2. MCP Resource Slot Inference (`mcp/adapter.py`)
New `infer_resource_slots()` static method on `MCPToolAdapter`:
- Scans MCP tool parameter schemas for file/directory/repo patterns
- Creates `ResourceSlot` objects with appropriate type, access mode, binding mode
- Mapping: `file_path`→file(rw), `directory`→directory(ro), `repo_path`→git-checkout(rw)
- Slots stored in `source_metadata["resource_slots"]` on registered `ToolSpec` objects

### Quality Gates

| Session | Result |
|---|---|
| `nox -s lint` | PASS |
| `nox -s typecheck` | PASS (0 errors) |
| `nox -s unit_tests` | PASS (10,817 scenarios) |
| `nox -s integration_tests` | PASS (6 new tests) |
| `nox -s coverage_report` | 97% (>= 97%) |

Closes #882

Reviewed-on: #964
Co-authored-by: Brent E. Edwards <brent.edwards@cleverthis.com>
Co-committed-by: Brent E. Edwards <brent.edwards@cleverthis.com>

benchmarks/checkpoint_rollback_bench.py

@@ -6,6 +6,9 @@ overhead across varying checkpoint counts.
 
 from __future__ import annotations
 
+import shutil
+import subprocess
+import tempfile
 from typing import ClassVar
 
 from cleveragents.application.services.checkpoint_service import CheckpointService
@@ -74,11 +77,27 @@ class RollbackSuite:
     """Benchmark rollback operations."""
 
     def setup(self) -> None:
-        """Prepare a service with sandbox and checkpoints."""
+        """Prepare a service with a real git sandbox and checkpoints."""
+        self._tmpdir = tempfile.mkdtemp(prefix="asv-rollback-")
+        subprocess.run(
+            ["git", "init", self._tmpdir],
+            check=True,
+            capture_output=True,
+        )
+        subprocess.run(
+            ["git", "commit", "--allow-empty", "-m", "initial"],
+            check=True,
+            capture_output=True,
+            cwd=self._tmpdir,
+        )
         self.svc = CheckpointService()
-        self.svc.register_sandbox(_PLAN_ID, "sandbox-path")
+        self.svc.register_sandbox(_PLAN_ID, self._tmpdir)
         self.cp = self.svc.create_checkpoint(_PLAN_ID, "commit-abc")
 
+    def teardown(self) -> None:
+        """Remove temporary sandbox directory."""
+        shutil.rmtree(self._tmpdir, ignore_errors=True)
+
     def time_rollback(self) -> None:
         """Time a single rollback operation."""
         self.svc.rollback_to_checkpoint(_PLAN_ID, self.cp.checkpoint_id)