v2.9.8 — gh_pushed data cleanup + CHANGELOG backfill

Changed

• Inventory gh_pushed cleanup: normalized 281 existing entries with gh_pushed: "N/A" → null, completing the v2.9.4 source fix. Deterministic / CI-reproducible (last_checked already migrated to integer by the pipeline).
• CHANGELOG: documented releases 2.9.2–2.9.8 in CHANGELOG.md (it had drifted to 2.5.8). The 2.6.0–2.9.1 gap is flagged in the file and remains available in Releases.

No code or site behavior change.

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