docs(execute_code): document project/strict execution modes (#12073)

Follow-up to PR #11971. Documents the new code_execution.mode config
key and what each mode actually does.

- user-guide/configuration.md: add mode: project to the yaml example,
  explain project vs strict and call out that security invariants are
  identical across modes.
- user-guide/features/code-execution.md: new 'Execution Mode' section
  with a comparison table and usage guidance; update the 'temporary
  directory' note so it reflects that script.py runs in the session
  CWD in project mode (staging dir stays on PYTHONPATH for imports);
  drop stale 'sandboxed' framing from the intro and skill-passthrough
  paragraph.
- getting-started/learning-path.md: update the one-line Code Execution
  summary to match (no longer 'sandboxed environments' — the default
  runs in the session's real working directory).

No code changes.

website/docs/user-guide/configuration.md

@@ -1104,14 +1104,22 @@ human_delay:
 
 ## Code Execution
 
-Configure the sandboxed Python code execution tool:
+Configure the `execute_code` tool:
 
 ```yaml
 code_execution:
+  mode: project                # project (default) | strict
   timeout: 300                 # Max execution time in seconds
   max_tool_calls: 50           # Max tool calls within code execution
 ```
 
+**`mode`** controls the working directory and Python interpreter for scripts:
+
+- **`project`** (default) — scripts run in the session's working directory with the active virtualenv/conda env's python. Project deps (`pandas`, `torch`, project packages) and relative paths (`.env`, `./data.csv`) resolve naturally, matching what `terminal()` sees.
+- **`strict`** — scripts run in a temp staging directory with `sys.executable` (Hermes's own python). Maximum reproducibility, but project deps and relative paths won't resolve.
+
+Environment scrubbing (strips `*_API_KEY`, `*_TOKEN`, `*_SECRET`, `*_PASSWORD`, `*_CREDENTIAL`, `*_PASSWD`, `*_AUTH`) and the tool whitelist apply identically in both modes — switching mode does not change the security posture.
+
 ## Web Search Backends
 
 The `web_search`, `web_extract`, and `web_crawl` tools support four backend providers. Configure the backend in `config.yaml` or via `hermes tools`: