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).

tests/hermes_cli/test_debug.py

@@ -291,9 +291,11 @@ class TestCaptureLogSnapshotRedaction:
         home = tmp_path / ".hermes"
         home.mkdir()
         monkeypatch.setenv("HERMES_HOME", str(home))
-        # Critical: ensure the user has NOT opted in to redaction. The whole
-        # point of this PR is that share-time redaction works for users who
-        # never set this env var.
+        # Baseline fixture: no explicit env-var opinion. With the post-#17691
+        # default of ON, the default-path tests below exercise the
+        # secure-default behaviour. The `force=True` regression test
+        # setenvs to "false" inline to prove force=True works even when
+        # the runtime flag is disabled.
         monkeypatch.delenv("HERMES_REDACT_SECRETS", raising=False)
 
         logs_dir = home / "logs"
@@ -324,21 +326,26 @@ class TestCaptureLogSnapshotRedaction:
         assert _REDACT_FIXTURE_TOKEN in snap.tail_text
         assert _REDACT_FIXTURE_TOKEN in (snap.full_text or "")
 
-    def test_force_true_overrides_unset_env_var(self, hermes_home_with_secret):
+    def test_force_true_works_when_redaction_disabled(
+        self, hermes_home_with_secret, monkeypatch
+    ):
         """Regression test: redact_sensitive_text short-circuits without force=True.
 
         If a future refactor drops `force=True` from `_redact_log_text`, this
         test fails immediately. Without `force=True`, the redactor returns the
-        input unchanged when HERMES_REDACT_SECRETS is unset, and the feature
-        ships silently broken for its target audience.
+        input unchanged when HERMES_REDACT_SECRETS=false, and the share-time
+        redaction feature ships silently broken for users who opted out of
+        runtime redaction (e.g. developers working on the redactor itself).
         """
         import os
 
+        # Force the runtime flag off so we're exercising the force=True path,
+        # not the default-on path.
+        monkeypatch.setenv("HERMES_REDACT_SECRETS", "false")
+
         from hermes_cli.debug import _capture_log_snapshot
 
-        # Belt-and-suspenders: confirm the env var is genuinely unset for this
-        # test so we know we're exercising the force=True path.
-        assert os.environ.get("HERMES_REDACT_SECRETS", "") == ""
+        assert os.environ.get("HERMES_REDACT_SECRETS", "") == "false"
 
         snap = _capture_log_snapshot("agent", tail_lines=10)