feat(security): enable secret redaction by default (#17691, #20785) (#21193)

Flip the default for HERMES_REDACT_SECRETS from off to on so the redactor
already wired into send_message_tool, logs, and tool output actually runs
on a fresh install.

- agent/redact.py: env-var default "" → "true"
- hermes_cli/config.py: DEFAULT_CONFIG security.redact_secrets True;
  two config-template comments rewritten
- gateway/run.py + cli.py: startup log / banner warning when the user
  has explicitly opted out, so the downgrade is visible in agent.log
  and at CLI banner time
- docs/reference/environment-variables.md: description reconciled
- tests: flipped the default-pin, restructured the force=True
  regression test to explicit-false instead of unset

Users who need raw credential values (redactor development) can still
opt out via security.redact_secrets: false in config.yaml or
HERMES_REDACT_SECRETS=false in .env.

Closes #17691.
Addresses #20785 (short-term output-pipeline recommendation).

agent/redact.py

@@ -56,12 +56,15 @@ _SENSITIVE_BODY_KEYS = frozenset({
 })
 
 # Snapshot at import time so runtime env mutations (e.g. LLM-generated
-# `export HERMES_REDACT_SECRETS=true`) cannot enable/disable redaction
-# mid-session.  OFF by default — user must opt in via
-# `security.redact_secrets: true` in config.yaml (bridged to this env var
-# in hermes_cli/main.py and gateway/run.py) or `HERMES_REDACT_SECRETS=true`
-# in ~/.hermes/.env.
-_REDACT_ENABLED = os.getenv("HERMES_REDACT_SECRETS", "").lower() in ("1", "true", "yes", "on")
+# `export HERMES_REDACT_SECRETS=false`) cannot disable redaction
+# mid-session.  ON by default — secure default per issue #17691. Users who
+# need raw credential values in tool output (e.g. working on the redactor
+# itself) can opt out via `security.redact_secrets: false` in config.yaml
+# (bridged to this env var in hermes_cli/main.py, gateway/run.py, and
+# cli.py) or `HERMES_REDACT_SECRETS=false` in ~/.hermes/.env. An opt-out
+# warning is logged at gateway and CLI startup so operators see the
+# downgrade — see `_log_redaction_status()` in gateway/run.py and cli.py.
+_REDACT_ENABLED = os.getenv("HERMES_REDACT_SECRETS", "true").lower() in ("1", "true", "yes", "on")
 
 # Known API key prefixes -- match the prefix + contiguous token chars
 _PREFIX_PATTERNS = [