refactor(config): add cfg_get() helper; migrate 20 nested-get call sites (#17304)

The "cfg.get('X', {}).get('Y', default)" pattern appears 50+ times
across tools/, gateway/, and plugins/. Each call site manually handles
the same three gotchas:

  1. Missing intermediate key → empty dict → chain works
  2. Non-dict value at intermediate position → AttributeError
     (uncaught in most sites, so a misconfigured YAML crashes the tool)
  3. cfg is None → AttributeError

Introduces cfg_get(cfg, *keys, default=None) in hermes_cli/config.py
as the canonical helper. Handles all three uniformly, returns default
only when the final key is *absent* (matches dict.get semantics —
explicit None values are preserved, falsy values like 0 / False / ''
are preserved).

Named cfg_get rather than cfg_path to avoid shadowing the existing
'cfg_path = _hermes_home / "config.yaml"' local variable that appears
in gateway/run.py, cron/scheduler.py, hermes_cli/main.py, etc.

Migrated 20 call sites as the first-batch proof-of-value:

  gateway/run.py            10 sites (agent/display subtrees)
  tools/browser_tool.py      3 sites
  tools/vision_tools.py      2 sites
  tools/browser_camofox.py   1 site
  tools/approval.py          1 site
  tools/skills_tool.py       1 site
  tools/skill_manager_tool.py 1 site
  tools/credential_files.py  1 site
  tools/env_passthrough.py   1 site

The remaining ~30 sites across plugins/ and smaller tool files can be
migrated opportunistically — the helper is now available and the
pattern is established.

Fixed a latent bug along the way: tools/vision_tools.py had its
cfg_get usage at line 560 inside a function that locally re-imports
'from hermes_cli.config import load_config', but the AST-based
migration script wrote the top-level cfg_get import to a different
function scope, leaving line 560's cfg_get as a NameError silently
swallowed by the surrounding try/except. Test
test_vision_uses_configured_temperature_and_timeout caught it. Fixed
by including cfg_get in the function-local import.

Verified:
- 7880/7893 tests/tools/ + tests/gateway/ + tests/hermes_cli/test_config
  tests pass; all 13 failures pre-existing on main (MCP, delegate,
  session_split_brain — verified earlier in the sweep).
- All 20 migrated sites AST-verified to have cfg_get in scope (either
  module-level or function-local).
- Live 'hermes chat' smoke: 2 turns + /model switch + tool calls +
  /quit, zero errors. Agent correctly counted 20 cfg_get hits across
  8 tool files — matching the migration.

Semantic parity verified against the original pattern across 8 edge
cases (missing keys, None values, falsy values, empty strings, string
instead of dict, None cfg, nested levels).

tools/skill_manager_tool.py

@@ -43,6 +43,7 @@ from hermes_constants import get_hermes_home, display_hermes_home
 from typing import Dict, Any, Optional, Tuple
 
 from utils import atomic_replace
+from hermes_cli.config import cfg_get
 
 logger = logging.getLogger(__name__)
 
@@ -66,7 +67,7 @@ def _guard_agent_created_enabled() -> bool:
     try:
         from hermes_cli.config import load_config
         cfg = load_config()
-        return bool(cfg.get("skills", {}).get("guard_agent_created", False))
+        return bool(cfg_get(cfg, "skills", "guard_agent_created", default=False))
     except Exception:
         return False