fix(config): retain last-known-good config when config.yaml fails to parse

[teknium1]

Summary

load_config() now retains the last successfully loaded config within a process when config.yaml fails to parse, instead of silently falling back to DEFAULT_CONFIG — which dropped every user override, including approvals.deny security rules that are documented to block commands even under --yolo.

Port of openai/codex#31188 ("Preserve managed exec policy after rules parse errors"). Codex's invariant: a parse failure in a policy-bearing config file must not replace the effective policy with an empty one. Their load_exec_policy_with_warning swapped the whole exec policy for Policy::empty() when a .rules file failed to parse, silently dropping managed prompt/forbidden rules. Hermes had the exact same bug shape in _load_config_impl().

The Hermes symptom: a long-running gateway whose user mid-edits config.yaml into broken YAML silently disarms their own approvals.deny rules (and every other override — fallback chain, aux providers, model settings) on the next load_config() cache miss. Reproduced live before the fix: with deny: ['curl*evil.com*'] loaded, corrupting the file mid-process made _match_user_deny_rule("curl http://evil.com/x") return None.

Changes

• hermes_cli/config.py _load_config_impl(): on parse failure, if _LAST_EXPANDED_CONFIG_BY_PATH holds a previously loaded config for this path, keep serving it. The retained config is cached under the corrupt file's signature so the broken file isn't re-parsed on every load; fixing the file changes the signature and triggers a normal reload. Fresh processes with no last-known-good keep the existing DEFAULT_CONFIG fallback unchanged.
• _warn_config_parse_failure(): new fallback= wording selector — the warning now says "Keeping the previously loaded config for this process — edits to config.yaml are being IGNORED until the YAML is fixed" when last-known-good is in play, instead of the misleading "Falling back to default config". Backup-to-.bak behavior unchanged.
• 5 new tests in tests/hermes_cli/test_config.py (retention incl. deny rules, recovery after fix, fresh-process default fallback preserved, no re-warn spam on cached loads).

Adaptation notes

• Codex preserves the managed policy layer specifically; Hermes' analog is the whole last-known-good merged config (user + managed), which _LAST_EXPANDED_CONFIG_BY_PATH already tracked for env-ref template preservation — no new state added.
• Codex keeps parse errors fatal for strict clients (codex exec); Hermes has no strict-client analog — all callers get the warn-and-retain behavior.
• save_config() stores the pre-expansion normalized dict in _LAST_EXPANDED_CONFIG_BY_PATH, so the retained value is passed through _expand_env_vars defensively (idempotent when already expanded).

Validation

	Before	After
deny rule after mid-process corruption	None (rule silently dropped)	still matches curl*evil.com*
fixed YAML after corruption	reloads	reloads (signature change busts retained cache)
fresh process + broken config	DEFAULT_CONFIG	DEFAULT_CONFIG (unchanged)
tests	—	149 passed test_config.py; 218 passed across config+deny-rule suites; 414 passed sibling _warn_config_parse_failure consumers

E2E (real imports, temp HERMES_HOME, real file I/O): 4/4 scenarios pass.

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