[Phase 1] Confirm Opus 4.8 standard pricing in model-pricing.tsv and lock it with a regression test

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Story

As a cost-reporting maintainer,
I want verify that claude-opus-4-8 resolves to the correct standard per-MTok rate in model-pricing.tsv and add a test that pins that behavior,
so that cost reports and ET multipliers stay accurate for the new model id without introducing the rejected Fast Mode rate.

Acceptance Criteria

1. It is confirmed (and recorded in the PR) that price_for claude-opus-4-8 <a-2026-date> resolves via the existing claude-opus-4-* glob (effective 2025-11-01) to 5.00 0.50 6.25 25.00 — the same standard rate that already prices claude-opus-4-7.
2. If, and ONLY if, an authoritative source (the Anthropic pricing page cited in model-pricing.tsv) shows the STANDARD (non-Fast-Mode) Opus 4.8 input/output rate differs from the wildcard, an explicit date-versioned claude-opus-4-8* row is APPENDED (never editing an existing row) with the verified rate; otherwise no row is added and the PR documents that the wildcard already prices it correctly.
3. No Fast Mode / express pricing ($10/$50 or any fast-mode surcharge) is added — consistent with model-pricing.tsv's stated convention that fast-mode surcharges are not modelled, and with the maintainer's rejection of the express tier.
4. A bats test in tests/model_pricing.bats asserts price_for claude-opus-4-8 (and et_multiplier_for claude-opus-4-8 = 5.0) returns the expected standard rate, mirroring the existing opus-4-7 cases, so the behavior is locked against future table edits.
5. bats tests/model_pricing.bats passes.

Tasks / Subtasks

• Verify claude-opus-4-8 price resolution against the existing wildcard and the cited authoritative pricing page; record the finding. (AC: test issue from agent #1, Go-live improvements for PR review agent #2)
• Append an explicit claude-opus-4-8* row ONLY if the authoritative standard rate differs from the wildcard; otherwise leave the table unchanged and document why. (AC: Go-live improvements for PR review agent #2, Add @claude delegation, auto-merge, and rebase handling #3)
• Add opus-4-8 cases to tests/model_pricing.bats (price_for + et_multiplier_for) mirroring the opus-4-7 cases. (AC: Optimize review: small-PR and incremental fast paths #4, feat: add Copilot engine support via REVIEW_ENGINE toggle #5)

Dev Notes

• Grounding: scripts/lib/model-pricing.tsv has the row claude-opus-4-* 2025-11-01 5.00 0.50 6.25 25.00, and the more-specific legacy rows claude-opus-4-1* / claude-opus-4-0* at $15. The selection rule is most-specific-glob then latest-effective_from (see the TSV header and price_for in scripts/lib/model-pricing.sh). claude-opus-4-8 matches only the claude-opus-4-* wildcard, so it already prices at $5/$0.50/$6.25/$25 — identical to how tests/model_pricing.bats line 15-16 asserts for claude-opus-4-7.
• Because the standard rate is already correct via the wildcard, the EXPECTED outcome is: no new pricing row, plus a regression test that locks opus-4-8 to the wildcard rate. Only append a row if the authoritative Anthropic pricing page shows a different STANDARD opus-4-8 rate — do not guess a price.
• The idea's $10/$50 figure is the Fast Mode rate, which this table explicitly does NOT model (see TSV header: 'fast-mode ... surcharges are NOT modelled') and which the maintainer rejected operationally. Do not introduce it.
• PRICE-CHANGE DISCIPLINE: never edit an existing row; APPEND a new effective-dated row (TSV header rule). Prices are data, reviewed as data (AGENTS.md cost reporting).
• Testing standards: extend tests/model_pricing.bats following the existing opus-4-7 test cases (price_for exact-string assertion + et_multiplier_for = 5.0). Run bats tests/model_pricing.bats.

Project Structure Notes

Touches only the pricing data table and its bats test; both are first-party. No engine or routing change.

References

• scripts/lib/model-pricing.tsv#claude-opus-4-wildcard
• scripts/lib/model-pricing.sh#price_for
• tests/model_pricing.bats#opus-4-7-cases
• AGENTS.md#cost-reporting

Likely target surface

• scripts/lib/model-pricing.tsv
• tests/model_pricing.bats

Story prepared by the BMAD Scrum Master (Bob) for epic #835. Status: ready-for-dev.