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).

tools/skills_tool.py

@@ -1146,6 +1146,26 @@ def skill_view(name: str, file_path: str = None, task_id: str = None) -> str:
         )
         setup_needed = bool(remaining_missing_required_envs)
 
+        # Register available skill env vars so they pass through to sandboxed
+        # execution environments (execute_code, terminal).  Only vars that are
+        # actually set get registered — missing ones are reported as setup_needed.
+        available_env_names = [
+            e["name"]
+            for e in required_env_vars
+            if e["name"] not in remaining_missing_required_envs
+        ]
+        if available_env_names:
+            try:
+                from tools.env_passthrough import register_env_passthrough
+
+                register_env_passthrough(available_env_names)
+            except Exception:
+                logger.debug(
+                    "Could not register env passthrough for skill %s",
+                    skill_name,
+                    exc_info=True,
+                )
+
         result = {
             "success": True,
             "name": skill_name,