Add Sign in with INK button brand assets (#237)

Signed-off-by: Jason Odoom <jason@adastracomputing.com>

Author: jasonodoom

README.md

@@ -159,7 +159,7 @@ src/           library implementation
   discovery/   Agent Card fetching and candidate-key extraction
   ink/         discovery gating, handshake budget, receipts, checkpointing
 specs/         protocol spec documents
-docs/          maturity notes, threat model, key rotation rules
+docs/          maturity notes, threat model, key rotation rules, brand assets
 test-vectors/  JSON interop vectors
 test/          vitest unit + integration tests
 ```

docs/brand/README.md

@@ -0,0 +1,22 @@
+# Sign in with INK button assets
+
+The "Sign in with INK" button that a relying party puts on its sign-in screen.
+It starts the flow in [`specs/ink-agent-authorization.md`](../../specs/ink-agent-authorization.md):
+a click hands the sign-in to the visitor's own INK agent, which confirms the
+request with them and proves their identity to the site.
+
+- `sign-in-with-ink-dark.svg` for light backgrounds, 196 by 44
+- `sign-in-with-ink-light.svg` for dark backgrounds, 196 by 44
+- `ink-nib.svg` for the nib mark on its own
+
+Drop one straight into a link:
+
+```html
+<a href="/auth/ink/start">
+  <img src="sign-in-with-ink-dark.svg" alt="Sign in with INK" width="196" height="44">
+</a>
+```
+
+Usage guidance, a live markup-and-CSS variant and the sizing rules are on the
+canonical brand page at
+<https://ink.tulpa.network/extensions/sign-in-button/>.

docs/brand/ink-nib.svg

@@ -145,6 +145,13 @@ another. This example shows the Sign in with INK flow, a different profile that
 composes the authorization grant. All three build on the same package surface and
 the same signing and verification primitives.
 
+## The button
+
+This example is the backend an RP runs. The control a user clicks to start the
+flow is the "Sign in with INK" button. Its assets live in
+[`docs/brand`](../../docs/brand), and the sizing, variants and usage rules are on
+the [brand page](https://ink.tulpa.network/extensions/sign-in-button/).
+
 ## Build and test
 
 ```sh