Refactor execpolicy fallback evaluation (#7544)

## Refactor of the `execpolicy` crate

To illustrate why we need this refactor, consider an agent attempting to
run `apple | rm -rf ./`. Suppose `apple` is allowed by `execpolicy`.
Before this PR, `execpolicy` would consider `apple` and `pear` and only
render one rule match: `Allow`. We would skip any heuristics checks on
`rm -rf ./` and immediately approve `apple | rm -rf ./` to run.

To fix this, we now thread a `fallback` evaluation function into
`execpolicy` that runs when no `execpolicy` rules match a given command.
In our example, we would run `fallback` on `rm -rf ./` and prevent
`apple | rm -rf ./` from being run without approval.

docs/config.md

@@ -603,7 +603,7 @@ metadata above):
 - `codex.tool_decision`
   - `tool_name`
   - `call_id`
-  - `decision` (`approved`, `approved_allow_prefix`, `approved_for_session`, `denied`, or `abort`)
+  - `decision` (`approved`, `approved_execpolicy_amendment`, `approved_for_session`, `denied`, or `abort`)
   - `source` (`config` or `user`)
 - `codex.tool_result`
   - `tool_name`