Prune stale atomic report temp files

[thomhurst]

Follow-up to #3831 review feedback.\n\n## Summary\n- prune owned atomic-write temp files older than 24 hours after successful history saves\n- retain recent temp files to avoid racing concurrent writers\n- leave unrelated temp files untouched\n- document the cleanup behavior\n\n## Validation\n- RunReportTests: 51 passed\n- focused post-main test: 1 passed\n- ModularPipelines.slnx Release build: 0 warnings, 0 errors

[thomhurst]

@coderabbitai review

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Scope reviewed: AtomicFileWriter.cs, FileSystemRunHistoryStore.cs, RunReportTests.cs, run-reports.md.

• Pattern anchoring (.modularpipelines-*.tmp) correctly matches only files this store owns; unrelated .tmp files are left alone.
• Staleness comparison (GetLastWriteTimeUtc <= UtcNow - 1 day) is directionally correct and UTC-consistent.
• Deletion happens only after a successful save, with IOException/UnauthorizedAccessException caught and logged per-file rather than failing the save.
• New test covers the three cases (stale owned, recent owned, unrelated) that matter for this change.
• No CLAUDE.md violations — changes stay within the core solution scope, no generated files touched, test placed correctly under test/ModularPipelines.UnitTests/.