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.

codex-rs/execpolicy/src/main.rs

@@ -1,6 +1,6 @@
 use anyhow::Result;
 use clap::Parser;
-use codex_execpolicy::ExecPolicyCheckCommand;
+use codex_execpolicy::execpolicycheck::ExecPolicyCheckCommand;
 
 /// CLI for evaluating exec policies
 #[derive(Parser)]
@@ -13,10 +13,6 @@ enum Cli {
 fn main() -> Result<()> {
     let cli = Cli::parse();
     match cli {
-        Cli::Check(cmd) => cmd_check(cmd),
+        Cli::Check(cmd) => cmd.run(),
     }
 }
-
-fn cmd_check(cmd: ExecPolicyCheckCommand) -> Result<()> {
-    cmd.run()
-}