fix(skill): harden mount/reload paths and HTTP errors against stale skill cache

The Box backends behave inconsistently when extra_mounts reference a
missing host directory (nsjail aborts the entire sandbox start, Docker
silently creates a root-owned empty dir on the host, E2B silently skips
the upload). The cache in skill_mgr.skills is only refreshed on
in-process mutations, so out-of-band changes — container rebuilds,
manual rm in the box volume, anything the LangBot API didn't drive —
leave a stale skill that later produces one of those bad mount paths.

- box/service.py: build_skill_extra_mounts now filters skills whose
  package_root is not isdir on the LangBot-visible filesystem and logs
  a warning, instead of passing the bad mount through to the backend
- skill/manager.py: reload_skills (Box path) drops skills whose
  package_root is missing on the LangBot-side filesystem before they
  reach the in-memory cache, with a summary warning
- api/http/controller/groups/skills.py: file/CRUD handlers now also
  catch BoxError (RuntimeError subclass, previously slipping past
  ``except ValueError`` and surfacing as 500); list/get handlers gain
  a try/except so a transient Box RPC failure becomes a clean 400
  instead of a stack trace

Tests added for build_skill_extra_mounts (skip missing, skip empty,
no skill manager) and SkillManager.reload_skills (drop missing on Box
path). Full unit suite: 279 passed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/langbot/pkg/box/service.py

@@ -192,6 +192,15 @@ class BoxService:
         This ensures that when a container is first created it already has
         all skill packages mounted, regardless of which skill is currently
         activated.
+
+        Skills whose ``package_root`` is missing or no longer a directory on
+        the LangBot-visible filesystem are skipped with a warning instead of
+        being passed through to the backend. Without this guard the three
+        backends behave inconsistently on a stale mount: nsjail refuses to
+        start the sandbox (failing every exec in the session), Docker
+        silently auto-creates a root-owned empty directory on the host, and
+        E2B silently skips the upload — none of which surfaces an
+        actionable error to the agent or operator.
         """
         skill_mgr = getattr(self.ap, 'skill_mgr', None)
         if skill_mgr is None:
@@ -205,6 +214,13 @@ class BoxService:
             package_root = str(skill_data.get('package_root', '') or '').strip()
             if not package_root:
                 continue
+            if not os.path.isdir(package_root):
+                self.ap.logger.warning(
+                    f'Skill "{skill_name}" package_root missing on filesystem '
+                    f'({package_root}); skipping mount to prevent sandbox failures. '
+                    f'The skill cache may be stale — consider reloading skills.'
+                )
+                continue
             mounts.append(
                 {
                     'host_path': package_root,