Move the config schema generator into a dedicated crate (#41915)

## What changed

- Add the `codex-config-schema` workspace and Bazel crate for the
  `codex-write-config-schema` binary.
- Preserve `codex-rs/core/config.schema.json` as the default output location.
- Update `just write-config-schema` to run the new crate and remove the schema
  generator's `clap` dependency and binary target from `codex-core`.

GitOrigin-RevId: 7447f97d6e44d1077b16d2d850f8a8add9117ea6

codex-rs/config-schema/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "codex-config-schema"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[[bin]]
+name = "codex-write-config-schema"
+path = "src/main.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+anyhow = { workspace = true }
+clap = { workspace = true, features = ["derive"] }
+codex-config = { workspace = true }