feat(UpstreamScan): expand scope to include file-level diffs across all four environments

[virtualian]

Summary

UpstreamScan currently only monitors GitHub activity (issues, PRs, discussions) on danielmiessler/Personal_AI_Infrastructure. It has no awareness of actual file-level differences between any of the four environments in the PAI stack.

Current scope

• Fetches open issues, PRs, discussions from upstream GitHub
• Relevance heuristics based on LOCAL_PATCHES.md filenames
• Digest-based tracking to avoid re-triaging old items

Missing scope

The tool has no ability to answer:

1. virtualian/pai vs upstream — What files in this repo diverge from danielmiessler/Personal_AI_Infrastructure main?
2. ~/projects/pai vs upstream — Same question at the local working copy level
3. ~/.claude/ (local install) vs upstream — What's actually installed locally that differs from upstream?
4. ~/.claude/ vs virtualian/pai — Is the local install in sync with this repo?

Proposed expansion

Add a --diff mode (or a companion RepoScan.ts tool) that:

• Compares file trees across the four environments
• Reports files that exist in upstream but not locally, and vice versa
• Reports files that exist in both but differ in content
• Scopes to relevant paths (e.g. .claude/, Packs/, Tools/)
• Outputs a structured report consumable by an LLM for triage

Context

• UpgradeCheck.ts does partial file diffing but only for files explicitly listed in LOCAL_PATCHES.md — not a full repo diff
• The Packs/ gap (upstream has 13 packs, this repo has 2) would have been caught by this