feat(secrets): proxy credential passthrough via config (#5)

[dsr-restyn]

Summary

Adds a config option for proxy-brokered credential placeholders (#4429). Sandboxed runtimes like NVIDIA OpenShell use credential brokering where env vars contain placeholders and a network proxy rewrites HTTP headers with real tokens on egress. Hermes strips these as if they were real secrets, breaking tools that depend on the placeholder being present.

Config

terminal:
  proxy_credentials:
    - SLACK_BOT_TOKEN
    - SLACK_APP_TOKEN

Listed vars bypass the provider-secret blocklist via the existing env_passthrough mechanism. Output redaction still applies. Registration is config-driven (not agent-controllable) and runs once, guarded by a module-level flag set after the try block completes.

Changes

File	Lines	What
hermes_cli/config.py	+5	proxy_credentials config key
tools/environments/local.py	+26	One-time registration in both _sanitize_subprocess_env and _make_run_env

31 additions, 0 deletions. No existing code modified.

Related Issues

Addresses #4429
Part of #410 (Secure Secrets Management umbrella)

Open to suggestions!