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/webhook.py

@@ -25,9 +25,8 @@ _SUBSCRIPTIONS_FILENAME = "webhook_subscriptions.json"
 
 
 def _hermes_home() -> Path:
-    return Path(
-        os.getenv("HERMES_HOME", str(Path.home() / ".hermes"))
-    ).expanduser()
+    from hermes_constants import get_hermes_home
+    return get_hermes_home()
 
 
 def _subscriptions_path() -> Path: