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).

website/docs/reference/environment-variables.md

@@ -456,7 +456,7 @@ Advanced per-platform knobs for throttling the outbound message batcher. Most us
 | `HERMES_EPHEMERAL_SYSTEM_PROMPT` | Ephemeral system prompt injected at API-call time (never persisted to sessions) |
 | `HERMES_PREFILL_MESSAGES_FILE` | Path to a JSON file of ephemeral prefill messages injected at API-call time. |
 | `HERMES_ALLOW_PRIVATE_URLS` | `true`/`false` — allow tools to fetch localhost/private-network URLs. Off by default in gateway mode. |
-| `HERMES_REDACT_SECRETS` | `true`/`false` — control secret redaction in logs and shareable outputs (default: `true`). |
+| `HERMES_REDACT_SECRETS` | `true`/`false` — control secret redaction in tool output, logs, and chat responses (default: `true`). |
 | `HERMES_WRITE_SAFE_ROOT` | Optional directory prefix that restricts `write_file`/`patch` writes; paths outside require approval. |
 | `HERMES_DISABLE_FILE_STATE_GUARD` | Set to `1` to turn off the "file changed since you read it" guard on `patch`/`write_file`. |
 | `HERMES_CORE_TOOLS` | Comma-separated override for the canonical core tool list (advanced; rarely needed). |