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

bot_bottle/manifest_agent.py

@@ -8,7 +8,7 @@ from typing import cast
 from .agent_provider import PROVIDER_TEMPLATES
 from .manifest_util import ManifestError, as_json_object
 from .manifest_git import ManifestGitUser
-from .manifest_schema import AGENT_MODEL_KEYS
+from .manifest_schema import AGENT_MODEL_KEYS, is_valid_entity_name
 
 
 @dataclass(frozen=True)
@@ -161,6 +161,16 @@ class ManifestAgent:
                         f"agent '{name}' skills[{i}] must be a string "
                         f"(was {type(skill).__name__})"
                     )
+                # Skill names become host/guest path segments and are
+                # interpolated into provisioning shell commands, so they
+                # must fit the same kebab-case convention as bottle/agent
+                # filenames — rejecting anything that could break out of a
+                # path segment or inject shell metacharacters.
+                if not is_valid_entity_name(skill):
+                    raise ManifestError(
+                        f"agent '{name}' skills[{i}] {skill!r} is not a valid "
+                        f"skill name; must match [a-z][a-z0-9-]*"
+                    )
                 collected.append(skill)
             skills = tuple(collected)