CI hygiene: bump deprecated action versions

[fcbond]

Summary

Small, low-risk cleanup found while auditing the build pipeline for the BergenTop page-registration fix (merged directly to main separately, since it was time-sensitive with the 2026 Bergen summit imminent):

• actions/checkout@v3 -> v4 and actions/cache@v3 -> v4 in the workflows (v3 majors are EOL and depend on the deprecated Node 16 runtime)
• buildindex custom action: node16 -> node20 runtime (no Node-16-specific API usage in index.js, so this should be a no-op behaviorally)
• .gitignore: add .claude/ so local Claude Code tool settings never get committed

Not included (flagged, not fixed)

• .github/actions/buildindex/node_modules/ is committed to the repo (482 files). Fixing that properly means bundling with @vercel/ncc into a single dist/index.js, which is a bigger change I didn't want to bundle into this low-risk PR.
• createdocs.py's get_change_text() builds a git log shell command by interpolating a wiki filename unescaped (subprocess.check_output([...], shell=True)). Wiki filenames come from user-editable page titles, so this is worth hardening even though it isn't currently known to be exploitable.
• The reusable workflow reads inputs via github.event.inputs.*; for workflow_call triggers the correct context is inputs.*. Works today, but is the wrong context and could silently misbehave on other trigger paths.

Test plan

• Confirm BuildDocs workflow still runs clean end-to-end on this branch (checkout/cache version bump shouldn't change behavior)
• Confirm the search index still builds correctly under Node 20 (buildindex action)

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