feat: add 3 new error entries (caching-artifacts x2, runner-environment)

[htekdev]

New Error Entries

ca-146: download-artifact v4/v5 Request Timeout from Blob Storage

• Category: caching-artifacts
• Severity: error
• Source: actions/download-artifact#249 (115 reactions)
• Error: \Unable to download artifact(s): Unable to download and extract artifact: Request timeout: /actions-results/...\
• SAS URL time window expires during large artifact downloads; intermittent, re-run usually fixes

ca-147: deploy-pages v4 HttpError No artifact_url Provided

• Category: caching-artifacts
• Severity: error
• Source: actions/deploy-pages#284 (34 reactions)
• Error: \Error: Creating Pages deployment failed\ + \Error: HttpError: No artifact_url provided\
• Using upload-pages-artifact v1/v2 with deploy-pages v4 causes HTTP 400; must upgrade both to matching major versions

re-489: checkout Fails with 'couldn't find remote ref refs/pull/N/merge' for Conflicting PRs

• Category: runner-environment
• Severity: error
• Source: Confirmed 0 DB matches via code search; very common Stack Overflow pattern
• Error: \atal: couldn't find remote ref refs/pull/123/merge\
• GitHub only creates the virtual merge ref for cleanly-mergeable PRs; PRs with conflicts or draft PRs missing the merge ref cause checkout to fail

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Research cycle: Strategy 2 round 250 (Stack Overflow [github-actions] top-voted questions). Exa/Perplexity over quota — used GitHub Issues API as fallback. All patterns verified with 0 DB matches via code search. Current main total after PRs #402+#403 merge: 1430 entries.