Auto-set subprocess env scrub when allowed_non_write_users is configured (#1093)

* Auto-set CLAUDE_CODE_SUBPROCESS_ENV_SCRUB when allowed_non_write_users is configured

Sets the env var automatically whenever allowed_non_write_users is
non-empty, so downstream workflows don't need to add it manually.
Updates the input description and docs/security.md to note the behavior.

🏠 Remote-Dev: homespace

* Fall back to inherited env when allowed_non_write_users is unset

🏠 Remote-Dev: homespace

* Let workflow/job env override the auto-set scrub flag

Env var takes priority so users can opt in/out via CLAUDE_CODE_SUBPROCESS_ENV_SCRUB
at job or workflow level independently of allowed_non_write_users.

🏠 Remote-Dev: homespace

docs/security.md

@@ -13,6 +13,7 @@
   - Accepts either a comma-separated list of specific usernames or `*` to allow all users
   - **Should be used with extreme caution** as it bypasses the primary security mechanism of this action
   - Is designed for automation workflows where user permissions are already restricted by the workflow's permission scope
+  - When set, Claude does a best-effort scrub of Anthropic, cloud, and GitHub Actions secrets from subprocess environments. This reduces but does not eliminate prompt injection risk — keep workflow permissions minimal and validate all outputs. Set `CLAUDE_CODE_SUBPROCESS_ENV_SCRUB: 0` in your workflow or job `env:` block to opt out.
 - **Token Permissions**: The GitHub app receives only a short-lived token scoped specifically to the repository it's operating in
 - **No Cross-Repository Access**: Each action invocation is limited to the repository where it was triggered
 - **Limited Scope**: The token cannot access other repositories or perform actions beyond the configured permissions