feat: mount skills directory into all remote backends with live sync (#3890)

Skills with scripts/, templates/, and references/ subdirectories need
those files available inside sandboxed execution environments. Previously
the skills directory was missing entirely from remote backends.

Live sync — files stay current as credentials refresh and skills update:
- Docker/Singularity: bind mounts are inherently live (host changes
  visible immediately)
- Modal: _sync_files() runs before each command with mtime+size caching,
  pushing only changed credential and skill files (~13μs no-op overhead)
- SSH: rsync --safe-links before each command (naturally incremental)
- Daytona: _upload_if_changed() with mtime+size caching before each command

Security — symlink filtering:
- Docker/Singularity: sanitized temp copy when symlinks detected
- Modal/Daytona: iter_skills_files() skips symlinks
- SSH: rsync --safe-links skips symlinks pointing outside source tree
- Temp dir cleanup via atexit + reuse across calls

Non-root user support:
- SSH: detects remote home via echo $HOME, syncs to $HOME/.hermes/
- Daytona: detects sandbox home before sync, uploads to $HOME/.hermes/
- Docker/Modal/Singularity: run as root, /root/.hermes/ is correct

Also:
- credential_files.py: fix name/path key fallback in required_credential_files
- Singularity, SSH, Daytona: gained credential file support
- 14 tests covering symlink filtering, name/path fallback, iter_skills_files

tools/environments/docker.py

@@ -315,7 +315,7 @@ class DockerEnvironment(BaseEnvironment):
         # Mount credential files (OAuth tokens, etc.) declared by skills.
         # Read-only so the container can authenticate but not modify host creds.
         try:
-            from tools.credential_files import get_credential_file_mounts
+            from tools.credential_files import get_credential_file_mounts, get_skills_directory_mount
 
             for mount_entry in get_credential_file_mounts():
                 volume_args.extend([
@@ -327,6 +327,20 @@ class DockerEnvironment(BaseEnvironment):
                     mount_entry["host_path"],
                     mount_entry["container_path"],
                 )
+
+            # Mount the skills directory so skill scripts/templates are
+            # available inside the container at the same relative path.
+            skills_mount = get_skills_directory_mount()
+            if skills_mount:
+                volume_args.extend([
+                    "-v",
+                    f"{skills_mount['host_path']}:{skills_mount['container_path']}:ro",
+                ])
+                logger.info(
+                    "Docker: mounting skills dir %s -> %s",
+                    skills_mount["host_path"],
+                    skills_mount["container_path"],
+                )
         except Exception as e:
             logger.debug("Docker: could not load credential file mounts: %s", e)