Task 11: User-Configurable Approval Predicates

This task is specific to codex-rs.

Status

General Status: Not started
Summary: Not started; missing Implementation details (How it was implemented and How it works).

Allow users to plug in an external executable that makes approval decisions for shell commands based on session context.

Support a new [[custom_approval_predicates]] section in config.toml, each with an executable = "..." field.
Before prompting the user, invoke each configured predicate in order, passing the following (via CLI args or env vars):
- Session ID
- Container working directory (CWD)
- Host working directory (CWD)
- Candidate shell command string
The predicate must print exactly one of allow, deny, or ask on stdout:
- allow → auto-approve and skip remaining predicates
- deny → auto-reject and skip remaining predicates
- ask → open the standard approval dialog and skip remaining predicates
If a predicate exits non-zero or outputs anything else, treat it as ask and continue to the next predicate.
Write unit and integration tests covering typical and edge-case predicate behavior.
Document configuration syntax and behavior in the top-level config docs (config.md).

How it was implemented
(Not implemented yet)

How it works
(Not implemented yet)

Consider passing context via environment variables (e.g. CODEX_SESSION_ID, CODEX_CONTAINER_CWD, CODEX_HOST_CWD, CODEX_COMMAND).
Reuse invocation logic from the auto-approval predicates feature (Task 02).