fix(skills): validate skill names and quote provisioning paths

Skill names become host/guest path segments interpolated into the
`bottle.exec` shell strings in each contrib provider's provision_skills.
They were validated only as strings, so a name with shell metacharacters
or path traversal could reach the command.

Layer two defenses:
  - Primary: reject any skill name that isn't kebab-case
    ([a-z][a-z0-9-]*) at manifest load, reusing the convention already
    enforced on bottle/agent filenames (new is_valid_entity_name helper
    in manifest_schema). Fails loud and early, protecting every consumer
    of the name — not just the exec call sites.
  - Failsafe: shlex.quote the interpolated skills_dir / dst paths in the
    claude, codex, and pi providers, so a future unvalidated field can't
    inject shell metacharacters even if it bypasses the load-time check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NkwFXLFff9PYPy4wgVBJp9

tests/unit/test_manifest_validation.py

@@ -165,6 +165,22 @@ class TestAgentValidation(unittest.TestCase):
         with self.assertRaises(ManifestError):
             ManifestAgent.from_dict("a", {"skills": [5]}, set())
 
+    def test_skill_name_rejects_shell_metacharacters(self) -> None:
+        # Skill names become host/guest path segments interpolated into
+        # provisioning shell commands; anything outside kebab-case is
+        # rejected at load so it can never reach a `bottle.exec` string.
+        for bad in ("foo; rm -rf /", "../escape", "foo bar", "Foo", "-leading"):
+            with self.assertRaises(ManifestError):
+                ManifestAgent.from_dict("a", {"skills": [bad]}, set())
+
+    def test_skill_name_accepts_kebab_case(self) -> None:
+        agent = ManifestAgent.from_dict(
+            "a", {"skills": ["init-entry", "quality-eval", "skill0"]}, set()
+        )
+        self.assertEqual(
+            agent.skills, ("init-entry", "quality-eval", "skill0")
+        )
+
     def test_prompt_not_string(self) -> None:
         with self.assertRaises(ManifestError):
             ManifestAgent.from_dict("a", {"prompt": 5}, set())