feat: env var passthrough for skills and user config (#2807)

* feat: env var passthrough for skills and user config

Skills that declare required_environment_variables now have those vars
passed through to sandboxed execution environments (execute_code and
terminal).  Previously, execute_code stripped all vars containing KEY,
TOKEN, SECRET, etc. and the terminal blocklist removed Hermes
infrastructure vars — both blocked skill-declared env vars.

Two passthrough sources:

1. Skill-scoped (automatic): when a skill is loaded via skill_view and
   declares required_environment_variables, vars that are present in
   the environment are registered in a session-scoped passthrough set.

2. Config-based (manual): terminal.env_passthrough in config.yaml lets
   users explicitly allowlist vars for non-skill use cases.

Changes:
- New module: tools/env_passthrough.py — shared passthrough registry
- hermes_cli/config.py: add terminal.env_passthrough to DEFAULT_CONFIG
- tools/skills_tool.py: register available skill env vars on load
- tools/code_execution_tool.py: check passthrough before filtering
- tools/environments/local.py: check passthrough in _sanitize_subprocess_env
  and _make_run_env
- 19 new tests covering all layers

* docs: add environment variable passthrough documentation

Document the env var passthrough feature across four docs pages:

- security.md: new 'Environment Variable Passthrough' section with
  full explanation, comparison table, and security considerations
- code-execution.md: update security section, add passthrough subsection,
  fix comparison table
- creating-skills.md: add tip about automatic sandbox passthrough
- skills.md: add note about passthrough after secure setup docs

Live-tested: launched interactive CLI, loaded a skill with
required_environment_variables, verified TEST_SKILL_SECRET_KEY was
accessible inside execute_code sandbox (value: passthrough-test-value-42).

website/docs/user-guide/features/skills.md

@@ -138,6 +138,8 @@ required_environment_variables:
 
 When a missing value is encountered, Hermes asks for it securely only when the skill is actually loaded in the local CLI. You can skip setup and keep using the skill. Messaging surfaces never ask for secrets in chat — they tell you to use `hermes setup` or `~/.hermes/.env` locally instead.
 
+Once set, declared env vars are **automatically passed through** to `execute_code` and `terminal` sandboxes — the skill's scripts can use `$TENOR_API_KEY` directly. For non-skill env vars, use the `terminal.env_passthrough` config option. See [Environment Variable Passthrough](/docs/user-guide/security#environment-variable-passthrough) for details.
+
 ## Skill Directory Structure
 
 ```text