Fix embed conversion prompt label for GitHub issues vs PRs

[nicepopo86-lang]

Summary\nThis fixes the GitHub URL conversion prompt so it reflects the pasted URL type:\n- -> Embed pull request\n- -> Embed issue\n\nThe previous prompt used a generic action and did not clarify issue vs PR intent. This addresses #1149's request for proper distinction in the embed popup flow.\n\n## Reproduction\n1. Open markdown editor.\n2. Paste a GitHub issue URL ().\n3. Observe conversion popup text.\n\n## Before\n- Prompt action text did not distinguish issue vs PR.\n\n## After\n- Prompt action text is context-aware ( / ).\n\n## Technical changes\n- Extended paste formatter result with optional .\n- Wired through menu callback.\n- Updated cloud editor, doc preview realtime hook path, and mobile doc editor to render the contextual label.\n\n## Validation\n- Static code review of conversion paths in cloud + mobile editor pipelines.\n- Dependency installation for local lint timed out in this environment; code changes are intentionally minimal and type-local.\n

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