Merge docs-watcher and diagram-renderer into one workflow

Replace the two separate workflows (gh-aw docs watcher + regular GHA
renderer) with a single native-input-docs.yml that handles both concerns.

A detect-changes job gates the two downstream jobs on what actually
changed: analyze-architecture runs when source/API files change,
render-diagrams runs when HTML diagram sources change.  Both can fire
on the same push if both categories are touched.

gh-aw was ruled out for the combined workflow because the framework
blocks contents:write, which the renderer needs to commit PNGs back.
The analysis job now calls the Anthropic API directly via Node.js,
keeping Claude in the loop without requiring the gh-aw toolchain.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: malmstein