Support SYSTEM skills. (#8220)

1. Remove PUBLIC skills and introduce SYSTEM skills embedded in the
binary and installed into $CODEX_HOME/skills/.system at startup.
2. Skills are now always enabled (feature flag removed).
3. Update skills/list to accept forceReload and plumb it through (not
used by clients yet).

codex-rs/Cargo.toml

@@ -141,6 +141,7 @@ icu_locale_core = "2.1"
 icu_provider = { version = "2.1", features = ["sync"] }
 ignore = "0.4.23"
 image = { version = "^0.25.9", default-features = false }
+include_dir = "0.7.4"
 indexmap = "2.12.0"
 insta = "1.44.3"
 itertools = "0.14.0"