docs(contributing): 5 recipes + decision flowchart for adding features

CONTRIBUTING.md closes the "how do I add anything" gap: the protocol PRD
defines the CONTRACT (what's allowed, what's verified), but until now
recipes were implicit — readers had to reverse-engineer patterns from
existing routes/vault_status.py + static/app.js + the STT migration.

Five recipes, one decision flowchart:
  A  Add a new /api/* endpoint
  B  Add a new UI element (CSS/JS/asset)
  C  Extend the loader API (add an 8th method) — STT migration is the example
  D  Add a config value
  E  Forced-internal escape hatch (use sparingly, discipline at 5 rows)

Each recipe: numbered steps, file paths, commit message template, test
requirement. Plus a PR checklist + quick-reference index.

README.md gets a top-of-page pointer so a new contributor lands on the
recipes within one click.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -4,6 +4,8 @@ THE single repo holding every Svrnty modification to `nesquena/hermes-webui`. Lo
 
 **Protocol contract:** [`hermes/docs/SVRNTY-PLUGIN-PROTOCOL.md`](../docs/SVRNTY-PLUGIN-PROTOCOL.md) — read this before contributing.
 
+**Adding features?** Read [`CONTRIBUTING.md`](CONTRIBUTING.md) for the 5 recipes + decision flowchart.
+
 ## Install
 
 ```bash