fix(selfhost): reconcile a stale AMS reporting export's schema on preserve-last-good

[JSONbored]

Summary

• Root-caused why the miner-usage Grafana dashboard hard-fails with "no such column: provider" on an engine-only self-host instance (no co-located miner): a genuine migration-drift bug in scripts/export-ams-reporting-db.sh, confirmed via git history -- PR fix(observability): avoid exposing live AMS ledgers #5471 first shipped this script with attempt_log_events' DDL missing provider/cost_usd/tokens_used; PR feat(miner): persist coding-agent provider + real cost on the attempt log #5637 added those columns to the DDL one day later.
• An instance whose ledger source has never existed creates its one-and-only export under whichever DDL was current at that moment. export_ledger's "source missing, output already exists" fail-open path then preserves that output forever -- the fingerprint/SCRIPT_VERSION rebuild check is unreachable while the source stays absent, so the later DDL widening was invisible to such an instance permanently.
• Fixed by reconciling the preserved output's schema against the current DDL's declared columns (ALTER TABLE ... ADD COLUMN) whenever a preserve-last-good path fires, mirroring packages/gittensory-miner/lib/attempt-log.js's own ensureOutcomeColumns pattern. Additive only -- existing rows and data are never touched; a genuinely-never-collected column is simply NULL for pre-existing rows.

Test plan

• npx tsc --noEmit
• npx tsx scripts/check-docs-drift.mjs
• shellcheck scripts/export-ams-reporting-db.sh
• New regression test: seeds a pre-feat(miner): persist coding-agent provider + real cost on the attempt log #5637-schema output file, runs the exporter with the source still missing, asserts the output's schema gets upgraded (columns added, NULL for the pre-existing row) rather than staying frozen -- verified this test fails without the fix (reverted the fix locally, confirmed the exact failure), and passes with it
• Full existing selfhost-ams-reporting.test.ts suite green (10/10)

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (12958f4) to head (6ece872).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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Flag	Coverage Δ
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shard-2	35.71% <ø> (ø)
shard-3	32.27% <ø> (ø)
shard-4	33.56% <ø> (ø)
shard-5	31.18% <ø> (+<0.01%)	⬆️
shard-6	44.64% <ø> (ø)

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