Proposal: footer and Codex usage extension examples

### What do you want to change?

Add two independently reviewable extension examples, each in its own PR:

1. A public-API-only two-line footer that keeps generic extension statuses at the right edge.
2. A standalone OpenAI Codex usage publisher that uses Senpi-resolved OAuth and `ctx.ui.setStatus()`, so it works with the built-in footer and custom footers.

No core behavior changes are proposed.

### Why?

The existing examples show `setFooter()` and `setStatus()` separately, but not a denser footer that composes arbitrary statuses or a provider-usage producer that stays footer-independent. These examples make both patterns copyable without coupling them.

### How?

Add isolated multi-file examples, package-level tests, README catalog entries, and `changes.md` records. The Codex example is OAuth-only, abortable/single-flight, rejects redirects, and documents that ChatGPT's usage endpoint is internal and best-effort.

Start by locating the existing setFooter() and setStatus() examples, package-level test conventions, README catalog, and changes.md. Add the two isolated examples with their tests and documentation records; done means both examples are independently reviewable and no core behavior changes are introduced.

Written by the indexing model from the issue text.