refactor: replace inline HERMES_HOME re-implementations with get_hermes_home()

16 callsites across 14 files were re-deriving the hermes home path
via os.environ.get('HERMES_HOME', ...) instead of using the canonical
get_hermes_home() from hermes_constants. This breaks profiles — each
profile has its own HERMES_HOME, and the inline fallback defaults to
~/.hermes regardless.

Fixed by importing and calling get_hermes_home() at each site. For
files already inside the hermes process (agent/, hermes_cli/, tools/,
gateway/, plugins/), this is always safe. Files that run outside the
process context (mcp_serve.py, mcp_oauth.py) already had correct
try/except ImportError fallbacks and were left alone.

Skipped: hermes_constants.py (IS the implementation), env_loader.py
(bootstrap), profiles.py (intentionally manipulates the env var),
standalone scripts (optional-skills/, skills/), and tests.

hermes_cli/plugins.py

@@ -38,6 +38,7 @@ from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any, Callable, Dict, List, Optional, Set, Union
 
+from hermes_constants import get_hermes_home
 from utils import env_var_enabled
 
 try:
@@ -258,8 +259,7 @@ class PluginManager:
         manifests: List[PluginManifest] = []
 
         # 1. User plugins (~/.hermes/plugins/)
-        hermes_home = os.environ.get("HERMES_HOME", os.path.expanduser("~/.hermes"))
-        user_dir = Path(hermes_home) / "plugins"
+        user_dir = get_hermes_home() / "plugins"
         manifests.extend(self._scan_directory(user_dir, source="user"))
 
         # 2. Project plugins (./.hermes/plugins/)