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/singularity.py

@@ -254,6 +254,28 @@ class SingularityEnvironment(BaseEnvironment):
         else:
             cmd.append("--writable-tmpfs")
 
+        # Mount credential files and skills directory (read-only).
+        try:
+            from tools.credential_files import get_credential_file_mounts, get_skills_directory_mount
+
+            for mount_entry in get_credential_file_mounts():
+                cmd.extend(["--bind", f"{mount_entry['host_path']}:{mount_entry['container_path']}:ro"])
+                logger.info(
+                    "Singularity: binding credential %s -> %s",
+                    mount_entry["host_path"],
+                    mount_entry["container_path"],
+                )
+            skills_mount = get_skills_directory_mount()
+            if skills_mount:
+                cmd.extend(["--bind", f"{skills_mount['host_path']}:{skills_mount['container_path']}:ro"])
+                logger.info(
+                    "Singularity: binding skills dir %s -> %s",
+                    skills_mount["host_path"],
+                    skills_mount["container_path"],
+                )
+        except Exception as e:
+            logger.debug("Singularity: could not load credential/skills mounts: %s", e)
+
         # Resource limits (cgroup-based, may require root or appropriate config)
         if self._memory > 0:
             cmd.extend(["--memory", f"{self._memory}M"])