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_cmd.py

@@ -16,6 +16,8 @@ import subprocess
 import sys
 from pathlib import Path
 
+from hermes_constants import get_hermes_home
+
 logger = logging.getLogger(__name__)
 
 # Minimum manifest version this installer understands.
@@ -26,8 +28,7 @@ _SUPPORTED_MANIFEST_VERSION = 1
 
 def _plugins_dir() -> Path:
     """Return the user plugins directory, creating it if needed."""
-    hermes_home = os.environ.get("HERMES_HOME", os.path.expanduser("~/.hermes"))
-    plugins = Path(hermes_home) / "plugins"
+    plugins = get_hermes_home() / "plugins"
     plugins.mkdir(parents=True, exist_ok=True)
     return plugins