feat: introduce codex_execpolicy crate for defining "safe" commands (#634)

As described in detail in `codex-rs/execpolicy/README.md` introduced in
this PR, `execpolicy` is a tool that lets you define a set of _patterns_
used to match [`execv(3)`](https://linux.die.net/man/3/execv)
invocations. When a pattern is matched, `execpolicy` returns the parsed
version in a structured form that is amenable to static analysis.

The primary use case is to define patterns match commands that should be
auto-approved by a tool such as Codex. This supports a richer pattern
matching mechanism that the sort of prefix-matching we have done to
date, e.g.:


5e40d9d221/codex-cli/src/approvals.ts (L333-L354)

Note we are still playing with the API and the `system_path` option in
particular still needs some work.

codex-rs/execpolicy/tests/cp.rs

@@ -0,0 +1,85 @@
+extern crate codex_execpolicy;
+
+use codex_execpolicy::get_default_policy;
+use codex_execpolicy::ArgMatcher;
+use codex_execpolicy::ArgType;
+use codex_execpolicy::Error;
+use codex_execpolicy::ExecCall;
+use codex_execpolicy::MatchedArg;
+use codex_execpolicy::MatchedExec;
+use codex_execpolicy::Policy;
+use codex_execpolicy::Result;
+use codex_execpolicy::ValidExec;
+
+fn setup() -> Policy {
+    get_default_policy().expect("failed to load default policy")
+}
+
+#[test]
+fn test_cp_no_args() {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &[]);
+    assert_eq!(
+        Err(Error::NotEnoughArgs {
+            program: "cp".to_string(),
+            args: vec![],
+            arg_patterns: vec![ArgMatcher::ReadableFiles, ArgMatcher::WriteableFile]
+        }),
+        policy.check(&cp)
+    )
+}
+
+#[test]
+fn test_cp_one_arg() {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &["foo/bar"]);
+
+    assert_eq!(
+        Err(Error::VarargMatcherDidNotMatchAnything {
+            program: "cp".to_string(),
+            matcher: ArgMatcher::ReadableFiles,
+        }),
+        policy.check(&cp)
+    );
+}
+
+#[test]
+fn test_cp_one_file() -> Result<()> {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &["foo/bar", "../baz"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "cp",
+                vec![
+                    MatchedArg::new(0, ArgType::ReadableFile, "foo/bar")?,
+                    MatchedArg::new(1, ArgType::WriteableFile, "../baz")?,
+                ],
+                &["/bin/cp", "/usr/bin/cp"]
+            )
+        }),
+        policy.check(&cp)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_cp_multiple_files() -> Result<()> {
+    let policy = setup();
+    let cp = ExecCall::new("cp", &["foo", "bar", "baz"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "cp",
+                vec![
+                    MatchedArg::new(0, ArgType::ReadableFile, "foo")?,
+                    MatchedArg::new(1, ArgType::ReadableFile, "bar")?,
+                    MatchedArg::new(2, ArgType::WriteableFile, "baz")?,
+                ],
+                &["/bin/cp", "/usr/bin/cp"]
+            )
+        }),
+        policy.check(&cp)
+    );
+    Ok(())
+}