Follow-up to PR #66479 salvage. The three new env vars
(HERMES_LOCAL_STREAM_STALE_TIMEOUT, HERMES_GATEWAY_MAX_STARTS,
HERMES_GATEWAY_START_WINDOW_S) were introduced as bare env-var reads with no
config.yaml surface or documentation — violating the .env-is-for-secrets-only
policy (behavioral settings must live in config.yaml, bridged to env internally).
- config.yaml: add gateway.respawn_storm {max_starts, window_seconds} to
DEFAULT_CONFIG, mirroring the existing restart_loop_guard pattern.
- gateway.py: read config.yaml first, env vars override as escape-hatch.
- environment-variables.md: document all three new env vars, noting the
config.yaml alternative for the gateway ones.
- chat_completion_helpers.py: cross-reference the env-var docs from the
local stale timeout comment.
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