Add update-patch-base skill for retargeting the patch installer

[jasonleenaylor]

Quick Summary

Adds a repo skill at .claude/skills/update-patch-base/SKILL.md capturing the procedure for retargeting the Patch Installer workflow to a new base build: verify the build-NNNN release contains BuildDir.zip and ProcRunner.zip, update the four defaults/fallbacks in .github/workflows/patch-installer-cd.yml, sweep for stray references, and smoke-test via workflow_dispatch with make_release: false.

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject <= 72 chars, no trailing punctuation; if body present, blank line then <= 80-char lines).
• No whitespace warnings locally (git diff --check --cached clean).
• Builds/tests pass locally (documentation-only change; no code touched).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate (no Src/** changes).

Notes for reviewers (optional)

Companion to #951, which performs the retarget this skill documents. The skill notes that base_release and base_build_number are separate inputs only for bootstrapping; whoever consolidates them after 9.3 is stable should update this skill in the same PR.

🤖 Generated with Claude Code

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This change is 

[jasonleenaylor]

Closing — opened prematurely; the skill will be PR'd when it's ready.