[Cloud Mode] Allow overriding the diff view base branch (currently locked to repo default branch)

Problem

In Claude Code Cloud Mode, the diff view always compares the current
working branch against the GitHub repository's default branch
(typically main). When a feature branch is created off a non-default
base — e.g. a release/YYYY-MM-DD branch — the diff view shows the
entire delta between that release branch and main, drowning out the
user's actual changes.

Terminal git diff works correctly because it honors the user's
explicit base. Only Cloud Mode's UI is affected.

Reproduction

1. Repo with default branch main
2. Cut a release/2025-01-15 branch from main, push it
3. Create a feature branch off release/2025-01-15
4. Make a 1-line change, open the branch in Cloud Mode
5. Cloud Mode's diff view shows hundreds of files (everything in

release/2025-01-15 vs main), not the 1-line change

Request

A per-session or per-repo setting to override the diff base branch.
Possible shapes:

• .claude/settings.json key like "diff.baseBranch": "release/2025-01-15"
• A CLAUDE.md directive
• A UI dropdown in Cloud Mode to pick the base for the current session
• Auto-detect from the branch's upstream / merge-base if set

Changing the GitHub repository default branch is not a viable
workaround because it affects the whole team (PR base, CI, branch
protection, Dependabot, etc.).

Environment

• Claude Code Cloud Mode (claude.ai/code), May 2026
• Anthropic Claude Opus 4.7