docs: point CLAUDE.md at api.astro, not the removed api.md (jbetancur#1395)

The API reference page was converted from `api.md` to `api.astro` in jbetancur#1350
(0dc7bec), but the CLAUDE.md instruction added earlier in jbetancur#1325 still names
`apps/docs/src/pages/docs/api.md`. That path no longer exists, so anyone
following CLAUDE.md when adding or changing a prop is sent to a missing file.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: hxperl

CLAUDE.md

@@ -50,7 +50,7 @@ The docs live in `apps/docs/src/pages/docs/`. Each feature has its own `.astro`
 **When adding a new prop or changing an existing one:**
 
 - Update the prop reference table on the relevant docs page.
-- Update the type signature in the API reference at `apps/docs/src/pages/docs/api.md`.
+- Update the type signature in the API reference at `apps/docs/src/pages/docs/api.astro`.
 
 **Nav and routing:**