fix: protect budget headroom accounting (#1905)

[svelderrainruiz]

Summary

• harden receipt discovery so private source snapshots do not block cost-rollup materialization
• surface whole-account remaining USD estimate and protected operational headroom in the GitHub budget hook
• mark operator spendable budget as unreconciled when labor timing is incomplete

Testing

• node --test tools/priority/tests/materialize-agent-cost-rollup.test.mjs tools/priority/tests/materialize-agent-cost-rollup-schema.test.mjs tools/priority/tests/github-comment-budget-hook.test.mjs tools/priority/tests/github-comment-budget-hook-schema.test.mjs
• node tools/npm/run-script.mjs schema:validate -- --schema docs/schemas/github-comment-budget-hook-report-v1.schema.json --data tests/results/_agent/cost/github-comment-budget-hook.json
• node tools/npm/run-script.mjs docs:manifest:validate
• git diff --check

[svelderrainruiz]

Published the budget correction slice behind #1937.

Highlights:

• filters private source snapshots out of cost-rollup materialization so normalized receipts can pass cleanly
• surfaces whole-account remaining estimate and protected operational headroom in the durable budget hook
• stops treating the $50,000 operator cap as immediately spendable when labor timing is still incomplete

Validated with focused materialization + hook tests, schema validation, docs manifest validation, and git diff --check.

Budget hook: blended lower bound $0.020100; operator $0.000000 of $50000.000000 cap (spendable remaining unreconciled; lower bound >=$50000.000000); window invoice-turn-2026-03-HQ1VJLMV-0029 spent $0.020100 remaining $49.979900; account est $166.000000 remaining from 4150 credits @ $0.040000 per credit; operational headroom $66.000000 (reserve-near); pressure tight; turns 1 total (0 live, 0 background); 1 turn(s) still pending labor timing; calibration reserve $100.000000 across 1 held window(s). Receipt: tests/results/_agent/cost/github-comment-budget-hook.json.

[svelderrainruiz]

Budget assumption confirmed by the operator:

• the earlier 2.9K remaining credits snapshot was pre-top-up
• HQ1VJLMV-0029 adds 1250 credits / $50

Confirmed state for this slice:

• $166 total remaining estimate
• $100 protected calibration reserve
• $66 operational headroom
• pressure tight

Budget hook: blended lower bound $0.020100; operator $0.000000 of $50000.000000 cap (spendable remaining unreconciled; lower bound >=$50000.000000); window invoice-turn-2026-03-HQ1VJLMV-0029 spent $0.020100 remaining $49.979900; account est $166.000000 remaining from 4150 credits @ $0.040000 per credit; operational headroom $66.000000 (reserve-near); pressure tight; turns 1 total (0 live, 0 background); 1 turn(s) still pending labor timing; calibration reserve $100.000000 across 1 held window(s). Receipt: tests/results/_agent/cost/github-comment-budget-hook.json.