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.

agent/model_metadata.py

@@ -510,8 +510,8 @@ def fetch_endpoint_model_metadata(
 
 def _get_context_cache_path() -> Path:
     """Return path to the persistent context length cache file."""
-    hermes_home = Path(os.environ.get("HERMES_HOME", Path.home() / ".hermes"))
-    return hermes_home / "context_length_cache.yaml"
+    from hermes_constants import get_hermes_home
+    return get_hermes_home() / "context_length_cache.yaml"
 
 
 def _load_context_cache() -> Dict[str, int]: