infra(git): rename leftovers: five files blocked by pre-existing debt, and rules the new branch model contradicts #16482

Sub-issue of #16461: what the in-repo name swap (#16479) could not, or should not, change mechanically.

1. Branch-name lines blocked by pre-existing debt in the same file

Each of these files still names the old branches. Changing even one line re-runs whole-file pre-commit gates, and those fail on older debt, which can't be baselined:

Fix each file's debt, then swap its branch names.

2. Rules whose reasons no longer hold

With main now both the default and the working branch, several rules and workflows explain themselves through the old split, where PRs targeted a non-default branch:

• .github/workflows/auto-close-issues.yml
• .github/workflows/dependabot-retarget.yml
• docs/developer/CI_BOT_ATTRIBUTION.md
• docs/developer/CLAUDE_WORKFLOW.md's rule that PRs into main don't auto-close, which is no longer true
• docs/developer/RELEASE_WORKFLOW.md's dependabot note

Each needs a decision: rewrite it for the new model, or retire it once shown to be redundant.

3. Naming choices to confirm

The release-sync head branch became release-sync-release (pipeline-scripts/release_sync_pull.py), and the workflow sync-main-to-release.yml. There's also a capitalised "Main" in scripts/pr-preflight.sh, which may be the ruleset's proper name.

Acceptance criteria

• None of the five files names Dev_new_gui, and their debt is fixed.
• Each rule in section 2 is rewritten for the new model or retired, with its reason.
• The naming choices in section 3 are confirmed or renamed.