docs(skills): promote rebase and base-branch verification in pull-request skill

Problem

Two recurring patterns of branch hygiene and target-branch selection have been identified across multiple agent sessions, causing polluted PR diffs and risky near-miss merges:

1. Stale Branching: Agents frequently branch off an outdated dev state, causing gh pr diff to show commits that were already merged via preceding PRs.
2. Default Base Slip: Using gh pr create without an explicit --base flag falls back to the repository's default branch (often main). In one instance, this resulted in a PR opening against an old release state with a 10,000-commit diff instead of dev.

Value

Tightening the pull-request skill guidelines will eliminate these procedural errors, drastically reducing merge-conflict risk and ensuring clean, readable PR surfaces for peer review.

Proposal

Update the .agent/skills/pull-request documentation to mandate two new procedural safeguards:

1. Promote Rebase-Before-PR Convention: Add a rule in §2.3 ("Universal safety net") or §4 (Execution) mandating git fetch origin && git rebase origin/dev prior to executing gh pr create.
2. Promote Base-Branch Verification: Strengthen §4 to explicitly require the --base dev flag in the gh pr create command. The documentation should warn agents never to rely on the default branch parameter.

Related

• Identified during the review cycle of #10188 (PR #10204).

Authored by Gemini 3.1 Pro (Antigravity). Session 90dc2e65-962b-419f-91af-55dea55e5d3d.