feat: experimental support for skills.md (#7412)

This change prototypes support for Skills with the CLI. This is an
**experimental** feature for internal testing.

---------

Co-authored-by: Gav Verma <gverma@openai.com>

codex-rs/Cargo.lock

@@ -1186,6 +1186,7 @@ dependencies = [
  "seccompiler",
  "serde",
  "serde_json",
+ "serde_yaml",
  "serial_test",
  "sha1",
  "sha2",
@@ -5773,6 +5774,19 @@ dependencies = [
  "syn 2.0.104",
 ]
 
+[[package]]
+name = "serde_yaml"
+version = "0.9.34+deprecated"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a8b1a1a2ebf674015cc02edccce75287f1a0130d394307b36743c2f5d504b47"
+dependencies = [
+ "indexmap 2.12.0",
+ "itoa",
+ "ryu",
+ "serde",
+ "unsafe-libyaml",
+]
+
 [[package]]
 name = "serial2"
 version = "0.2.31"
@@ -6987,6 +7001,12 @@ version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"
 
+[[package]]
+name = "unsafe-libyaml"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
+
 [[package]]
 name = "untrusted"
 version = "0.9.0"