execpolicy: add host_executable() path mappings (#12964)

## Why

`execpolicy` currently keys `prefix_rule()` matching off the literal
first token. That works for rules like `["/usr/bin/git"]`, but it means
shared basename rules such as `["git"]` do not help when a caller passes
an absolute executable path like `/usr/bin/git`.

This PR lays the groundwork for basename-aware matching without changing
existing callers yet. It adds typed host-executable metadata and an
opt-in resolution path in `codex-execpolicy`, so a follow-up PR can
adopt the new behavior in `unix_escalation.rs` and other call sites
without having to redesign the policy layer first.

## What Changed

- added `host_executable(name = ..., paths = [...])` to the execpolicy
parser and validated it with `AbsolutePathBuf`
- stored host executable mappings separately from prefix rules inside
`Policy`
- added `MatchOptions` and opt-in `*_with_options()` APIs that preserve
existing behavior by default
- implemented exact-first matching with optional basename fallback,
gated by `host_executable()` allowlists when present
- normalized executable names for cross-platform matching so Windows
paths like `git.exe` can satisfy `host_executable(name = "git", ...)`
- updated `match` / `not_match` example validation to exercise the
host-executable resolution path instead of only raw prefix-rule matching
- preserved source locations for deferred example-validation errors so
policy load failures still point at the right file and line
- surfaced `resolvedProgram` on `RuleMatch` so callers can tell when a
basename rule matched an absolute executable path
- preserved host executable metadata when requirements policies overlay
file-based policies in `core/src/exec_policy.rs`
- documented the new rule shape and CLI behavior in
`execpolicy/README.md`

## Verification

- `cargo test -p codex-execpolicy`
- added coverage in `execpolicy/tests/basic.rs` for parsing, precedence,
empty allowlists, basename fallback, exact-match precedence, and
host-executable-backed `match` / `not_match` examples
- added a regression test in `core/src/exec_policy.rs` to verify
requirements overlays preserve `host_executable()` metadata
- verified `cargo test -p codex-core --lib`, including source-rendering
coverage for deferred validation errors

codex-rs/execpolicy/src/lib.rs

@@ -2,6 +2,7 @@ pub mod amend;
 pub mod decision;
 pub mod error;
 pub mod execpolicycheck;
+mod executable_name;
 pub mod parser;
 pub mod policy;
 pub mod rule;
@@ -18,6 +19,7 @@ pub use error::TextRange;
 pub use execpolicycheck::ExecPolicyCheckCommand;
 pub use parser::PolicyParser;
 pub use policy::Evaluation;
+pub use policy::MatchOptions;
 pub use policy::Policy;
 pub use rule::NetworkRuleProtocol;
 pub use rule::Rule;