feat: add 1 new error entry (caching-artifacts) — 2026-06-21

[htekdev]

New error entries — June 21, 2026 cycle (Sunday)

ca-156: actions/cache save silently fails with misleading 'Unable to reserve cache with key' message when token has read-only cache access

• Source: actions/cache PR #1768 (June 18, 2026) + actions/toolkit PR #2435
• Category: caching-artifacts | Severity: silent-failure
• What's new: Documents the deceptive "Unable to reserve cache with key X, another job may be creating this cache" warning that appears when the GITHUB_TOKEN has read-only cache access — currently affects fork pull_request workflows, and will expand to pull_request_target with an upcoming platform change (June 2026+). The message misleads developers into investigating concurrency races when the real cause is a permission denial. Fix: upgrade to actions/cache@v5.1.0+ which emits a correct "cache write denied" message instead.

Validation pass: 8 entries — all accurate, no corrections needed

• ys-121 (uses: no expressions) ✅
• ys-118 (setup-python matrix 3.10 float) ✅
• caching-artifacts-121 (upload-artifact v7 409 conflict) ✅
• permissions-auth-012 (reusable workflow permissions) ✅
• permissions-auth-063 (security-events write for SARIF) ✅
• triggers-026 (workflow_run skipped when trigger filtered) ✅
• concurrency-timing-055 (head_branch null for schedule/dispatch) ✅
• known-unsolved-107 (workflow_run can't trigger workflow_call-only) ✅

[htekdev]

✅ Approved by Hector for agent merge at 2026-06-23T19:03:59.495Z

The Rocha merge-agent will rebase this PR onto the latest main, wait for CI, and merge when green. This approval persists across rebases.