fix(security): H1 — replace MD5 with SHA-256 in config/skill watchers

Both watcher.py (ConfigWatcher) and skill_loader/watcher.py
(SkillsWatcher) used hashlib.md5() for file-integrity change detection.
MD5 is collision-prone: a crafted config file could produce the same
hash as a benign one, silently suppressing the hot-reload callback and
preventing agents from picking up legitimate config changes.

Replace hashlib.md5 → hashlib.sha256 in both _hash_file() methods.
Update docstrings, comments, and the type-annotation comment
(rel_path → md5 hex → sha256 hex).

Test update: test_skills_watcher.py — rename helper _md5 → _sha256,
update the hash-length assertion from 32 (MD5) to 64 (SHA-256), and
rename the test from test_hash_file_returns_md5_for_existing_file to
test_hash_file_returns_sha256_for_existing_file. All 25 watcher tests
pass.

Note: H2 (a2a_client.py timeout=None) was already fixed in Cycle 5
(timeout=httpx.Timeout(connect=30.0, read=300.0, ...)) — confirmed by
code review before opening this PR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

workspace-template/skill_loader/watcher.py

@@ -7,7 +7,7 @@ Architecture
 ------------
 ``SkillsWatcher`` runs as a background asyncio task alongside the agent.  It
 polls the skill directories every ``POLL_INTERVAL`` seconds (default 3 s),
-computes MD5 hashes of every file, and fires ``_reload_skill()`` when any
+computes SHA-256 hashes of every file, and fires ``_reload_skill()`` when any
 file inside a skill's folder changes.
 
 ``_reload_skill()`` calls ``load_skills()`` from ``skills.loader`` for the
@@ -71,7 +71,7 @@ class SkillsWatcher:
         self.config_path = config_path
         self.skill_names = list(skill_names)
         self.on_reload   = on_reload
-        self._hashes: dict[str, str] = {}   # rel_path → md5 hex
+        self._hashes: dict[str, str] = {}   # rel_path → sha256 hex
         self._running = False
 
     # ------------------------------------------------------------------
@@ -103,12 +103,13 @@ class SkillsWatcher:
 
     def _hash_file(self, path: Path) -> str:
         try:
-            return hashlib.md5(path.read_bytes()).hexdigest()
+            # H1: SHA-256 replaces MD5 for file-integrity change detection.
+            return hashlib.sha256(path.read_bytes()).hexdigest()
         except OSError:
             return ""
 
     def _scan(self) -> dict[str, str]:
-        """Return {relative_path: md5} for every file in watched skill dirs."""
+        """Return {relative_path: sha256} for every file in watched skill dirs."""
         hashes: dict[str, str] = {}
         root = self._skills_root()
         for skill_name in self.skill_names:

workspace-template/tests/test_skills_watcher.py

@@ -40,8 +40,8 @@ def _write(path: Path, content: str) -> None:
     path.write_text(content, encoding="utf-8")
 
 
-def _md5(content: str) -> str:
-    return hashlib.md5(content.encode()).hexdigest()
+def _sha256(content: str) -> str:
+    return hashlib.sha256(content.encode()).hexdigest()
 
 
 # ============================================================================
@@ -149,7 +149,7 @@ class TestChangedSkills:
         _write(tmp_path / "skills" / "unwatched" / "SKILL.md", "v2")
         # Also add path for unwatched to new_hashes manually (shouldn't matter)
         new_hashes = w._scan()
-        new_hashes["unwatched/SKILL.md"] = _md5("v2")
+        new_hashes["unwatched/SKILL.md"] = _sha256("v2")
 
         changed = w._changed_skills(new_hashes)
         assert "unwatched" not in changed
@@ -351,13 +351,13 @@ class TestHashFile:
         result = w._hash_file(missing)
         assert result == ""
 
-    def test_hash_file_returns_md5_for_existing_file(self, tmp_path):
-        """_hash_file returns a non-empty hex digest for a readable file."""
+    def test_hash_file_returns_sha256_for_existing_file(self, tmp_path):
+        """_hash_file returns a non-empty SHA-256 hex digest for a readable file."""
         f = tmp_path / "real_file.txt"
         f.write_text("hello")
         w = SkillsWatcher(str(tmp_path), [])
         result = w._hash_file(f)
-        assert len(result) == 32  # MD5 hex digest length
+        assert len(result) == 64  # SHA-256 hex digest length
         assert result != ""
 
 

workspace-template/watcher.py

@@ -37,7 +37,11 @@ class ConfigWatcher:
 
     def _hash_file(self, path: str) -> str:
         try:
-            return hashlib.md5(Path(path).read_bytes()).hexdigest()
+            # H1: SHA-256 replaces MD5 for file-integrity change detection.
+            # MD5 is collision-prone; using SHA-256 prevents a crafted config
+            # file from producing the same hash as a benign one, which would
+            # silently suppress the hot-reload callback.
+            return hashlib.sha256(Path(path).read_bytes()).hexdigest()
         except (OSError, IOError):
             return ""