feat: Ability to update the branch description

[Dhanus3133]

Why

Allow to update the branch description from the UI

Closes #1045

What changed

• Added inline editing of branch descriptions on the branch detail page, which uses the BranchUpdate mutation. Includes edit/cancel/save flow with keyboard support (Enter/Escape)

How to review

Screen.Recording.2026-05-30.at.2.34.10.PM.mov

pnpm test branch-edit-description

Checklist

• Tests added/updated
• Changelog entry added (uv run towncrier create ...)
• External docs updated (if user-facing or ops-facing change)
• Internal .md docs updated (internal knowledge and AI code tools knowledge)
• I have reviewed AI generated content

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Summary by cubic

Adds inline editing for branch descriptions on the branch details page. Authenticated users can edit non‑default branches, with changes saved via GraphQL and reflected across lists immediately.

• New Features
  • Introduced BranchEditDescription with edit/cancel/save and Enter/Escape support.
  • Integrated into branch details; shows "—" when empty, 1000‑char limit; edit icon hidden when not allowed.
  • Added BranchUpdate GraphQL mutation and domain updateBranch for name/description.
  • @tanstack/react-query mutation updates store, syncs list/detail caches, and invalidates related queries.

^{Written for commit 9202e43. Summary will update on new commits.}