chore(skills): optimize updating and updating-checksums reference docs

[jdalton]

Summary

• Trim updating-checksums/reference.md from 253 → ~80 lines: remove stale release asset filename patterns, collapse duplicate command/output sections, tighten troubleshooting
• Trim updating/reference.md from 229 → ~70 lines: remove monorepo structure diagram (derivable from repo), collapse redundant npm update detail, tighten CI/interactive mode table

Changes follow Claude agent skills best practices:

• Concise reference files (context window is a shared resource)
• No time-sensitive content (asset filename patterns go stale)
• No information Claude already knows (monorepo layout, basic pnpm commands)
• Progressive disclosure: SKILL.md stays as the entry point, reference.md covers only what isn't obvious

Test plan

• Skills remain functionally complete — all commands, edge cases, and troubleshooting preserved

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Note

Low Risk
Low risk documentation-only change; no runtime code paths or build logic are modified.

Overview
Adds .claude/skills/updating/reference.md and .claude/skills/updating-checksums/reference.md to document the expected workflows for npm dependency updates (pnpm run update) and external tool checksum syncing (sync-checksums.mjs), including command examples and when to run each step.

Includes concise guidance on checksum sources/formats, GitHub-release vs non-GitHub tool handling, and troubleshooting for common sync failures (rate limits, missing releases, JSON validation, large downloads).

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