Cloud telemetry: produce complete per-PR spend coverage observations

[jeffhuber]

Problem

Code Mower already records cost and token metadata when providers expose it, and pr_outcome supports expected-versus-reported run counts. In practice, repository syncs do not consistently join builder and reviewer attempts into a complete per-PR cost-coverage observation, so CodeMower.com correctly refuses to publish cost per PR.

Outcome

Produce defensible pr_outcome cost coverage from local builder/reviewer ledgers and GitHub outcome truth, while preserving unknown cost as unknown.

Scope

• Join metadata-only builder and reviewer attempts to one repository/PR identity.
• Count expected and cost-reported attempts using final configured lane posture and observed builder provenance.
• Emit cost_coverage=complete, partial, or unknown plus expected/reported counts and observed total cost only when justified.
• Add a safe coverage diagnostic showing missing provider/lane sources without exposing commands, paths, auth output, or secrets.
• Keep provider-reported cost authoritative; never infer dollars from tokens unless an explicit versioned pricing source is configured.
• Update the cloud contract, repo-sync/dogfood behavior, docs, fixtures, and tests.

Acceptance criteria

• Complete, partial, and unknown examples produce the documented v1 pr_outcome shape.
• Missing cost is never serialized as zero.
• Duplicate uploads are idempotent and do not double-count attempts.
• Existing clients and CodeMower.com remain backward-compatible.
• CodeMower.com's paired issue is linked before merge.

Process

One focused PR, tests and docs, independent Codex and Claude final-head audits, required green gate, and metadata-only upload.

[jeffhuber]

Paired CodeMower.com consumer issue: https://github.com/jeffhuber/codemower.com/issues/241

The OSS producer must land first. The server remains backward-compatible, and missing provider cost remains unknown rather than zero.