docs(bdd): scenarios for paste-a-URL + onboarding burst, mapped to tests

Adds paste_url.feature (valid/invalid/not-found/dedup, +@pending no-captions)
and an onboarding-burst scenario on connect; all non-pending scenarios mapped in
the coverage gate to their existing Go tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/use-cases/paste_url.feature

@@ -0,0 +1,30 @@
+Feature: Paste a YouTube URL to summarize any video
+  As a user
+  I want to paste a YouTube link and get a summary
+  So that I can pull the specific video I want now, even from channels I don't follow
+
+  Scenario: Paste a valid YouTube URL
+    Given I am connected
+    When I paste a valid YouTube video URL
+    Then the video is added to my feed scoped to me
+    And it is queued for summarization through the shared rate gate
+
+  Scenario: Pasting an invalid link is rejected
+    When I paste something that is not a YouTube video URL
+    Then I get a clear error and nothing is added
+
+  Scenario: Pasting a video that cannot be found is honest
+    When I paste a URL whose video cannot be found
+    Then I am told it couldn't be found and nothing is added
+
+  Scenario: Pasting the same video twice does not duplicate it
+    Given I have pasted a video
+    When I paste the same video again
+    Then my feed still has exactly one entry for it
+
+  @pending
+  # Covered by the engine's ADR-010 no-transcript terminal state (degrade-never-error);
+  # there is no paste-specific test for it.
+  Scenario: A pasted video with no captions resolves honestly
+    When I paste a video that has no captions
+    Then it resolves to the "no transcript available" terminal state