fix(auth): resolve API keys from ~/.hermes/.env and credential_pool

_resolve_api_key_provider_secret() and _seed_from_env() only checked
os.environ for provider API keys. When keys exist in ~/.hermes/.env but
are not loaded into the process environment (e.g. ACP adapter entry
point, post-session-start .env edits, or non-CLI entry points), the
resolution returns an empty string, causing HTTP 401 failures.

Changes:
- credential_pool._seed_from_env: use get_env_value() which checks both
  os.environ and ~/.hermes/.env file, preventing _prune_stale_seeded_entries
  from removing valid entries whose env var isn't in os.environ
- credential_pool._seed_from_env: same fix for openrouter and
  base_url_env_var resolution
- auth._resolve_api_key_provider_secret: use get_env_value() instead of
  os.getenv(), and add credential_pool fallback when env resolution fails

Fixes #15914

agent/credential_pool.py

@@ -1273,7 +1273,12 @@ def _seed_from_env(provider: str, entries: List[PooledCredential]) -> Tuple[bool
         def _is_source_suppressed(_p, _s):  # type: ignore[misc]
             return False
     if provider == "openrouter":
-        token = os.getenv("OPENROUTER_API_KEY", "").strip()
+        # Check both os.environ and ~/.hermes/.env file
+        try:
+            from hermes_cli.config import get_env_value
+            token = (get_env_value("OPENROUTER_API_KEY") or "").strip()
+        except Exception:
+            token = os.getenv("OPENROUTER_API_KEY", "").strip()
         if token:
             source = "env:OPENROUTER_API_KEY"
             if _is_source_suppressed(provider, source):
@@ -1299,7 +1304,11 @@ def _seed_from_env(provider: str, entries: List[PooledCredential]) -> Tuple[bool
 
     env_url = ""
     if pconfig.base_url_env_var:
-        env_url = os.getenv(pconfig.base_url_env_var, "").strip().rstrip("/")
+        try:
+            from hermes_cli.config import get_env_value
+            env_url = (get_env_value(pconfig.base_url_env_var) or "").strip().rstrip("/")
+        except Exception:
+            env_url = os.getenv(pconfig.base_url_env_var, "").strip().rstrip("/")
 
     env_vars = list(pconfig.api_key_env_vars)
     if provider == "anthropic":
@@ -1310,7 +1319,12 @@ def _seed_from_env(provider: str, entries: List[PooledCredential]) -> Tuple[bool
         ]
 
     for env_var in env_vars:
-        token = os.getenv(env_var, "").strip()
+        # Check both os.environ and ~/.hermes/.env file
+        try:
+            from hermes_cli.config import get_env_value
+            token = (get_env_value(env_var) or "").strip()
+        except Exception:
+            token = os.getenv(env_var, "").strip()
         if not token:
             continue
         source = f"env:{env_var}"