Validate missing-cost evidence and finite explanation totals

[hudsonaikins]

Cost explanations now require missing actual-cost evidence to directly match the supplied unavailable reason. A negated reason or a reason appearing only in a forecast clause is rejected. Accumulated contributions must remain finite, even when individual inputs are finite.

Includes operator-owned acceptance regressions, CLI JSON checks, and documentation. PR #64 has merged; this candidate is integrated against main and retains its synthetic-confidence fix.

Validation on 3ceeb3d534dcc343c0cb08f19b1178eb5772249e:

• Focused variance/diff/explain and operator acceptance tests pass.
• Exact-commit local Tabellio result validation-725987d1-2030-4c6f-b843-e031fb6929bb passed all six required validators with zero provider cost.
• All six hosted checks passed: dead-code, verify-go, validate, verify-install-smoke, verify-website, and security-scan.
• Codex reviewed the final changes; no GitHub review threads or requested changes remain.

Tracking: PCTL-17 product validation. Original AgentShift story 5fdc67af-1610-48ab-ad4c-c783a522d572, run 7b01c7fb1c9fb686, and candidate 4c1f7879ec596475131f15d7638efb016e3a4ca8 remain preserved in history. Owner authorized integration and merge. No external release, provider calls, or website deployment.

[makeplane]

Linked to Plane Work Item(s)

References

• (PCTL-17) Adopt exact-head product validation gate

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