test: diagnose intermittent JSONL inventory catalog preparation failure

Problem

TestUsableAutoSkillsStockBuildPolicyMatrix/explicit_evaluated failed on an otherwise unrelated CI-only perf-workflow PR with:

jsonlstore: inventory changed repeatedly while preparing catalog

Observed in PR #1222, CI run 34153750204. The PR changes only .github/workflows/perf.yml, so it cannot directly change the learning or JSONL-store execution path.

Relevant path

Store.readyInventoryCatalog makes up to three attempts to observe a stable inventory fingerprint around discoveryMetaListLocked, then returns the error above if the inventory keeps changing:

• internal/adapter/store/jsonlstore/metalist.go:281-310
• internal/app/usable_auto_skills_matrix_e2e_test.go (TestUsableAutoSkillsStockBuildPolicyMatrix)

The test's diagnostics show the stale-session sweeper's list operation failed with this error, after which the expected learned skill was absent.

Related work

#1088 tracks a different intermittent failure in the same matrix test (learned skill operation failed) and suspected skill-store publication contention. This report records the distinct, actionable JSONL inventory-catalog failure path.

Acceptance criteria

• Reproduce or otherwise identify the concurrent inventory mutation that exhausts the three-attempt catalog preparation loop during this test.
• Determine whether the retry policy is insufficient or a test/production synchronization defect exists.
• Preserve fail-closed cursor and inventory consistency semantics.
• Add a deterministic regression test once the source is identified.