Proof output retention Phase 2: upstream prompt policy and budget preflight

[tonyketcham]

Summary

Implement honest prompt-budget controls for full/summarized upstream transcript use.

Source Context

• Cursor cloud-agent follow-up: https://cursor.com/agents/bc-51322132-f3a3-4f80-b0cd-0ed319ccce62
• Implementation PR: docs: plan proof output retention #199
• Proposal artifact: https://github.com/FlatbreadLabs/flatbread/blob/toeknee/proof-output-retention-ce62/docs/proposals/proof-output-retention-plan.md
• Review artifact: https://github.com/FlatbreadLabs/flatbread/blob/toeknee/proof-output-retention-ce62/docs/proposals/proof-output-retention-review.md
• Judge artifact: https://github.com/FlatbreadLabs/flatbread/blob/toeknee/proof-output-retention-ce62/docs/proposals/proof-output-retention-judge.md
• Phase-specific artifact section: https://github.com/FlatbreadLabs/flatbread/blob/toeknee/proof-output-retention-ce62/docs/proposals/proof-output-retention-plan.md#L232-L255

Why This Exists

• Phase 1 splits execution-authoritative transcripts from bounded canvas display, but it intentionally leaves prompt-budget controls narrow.
• The proposal says Phases 1 and 2 together carry the full 'remove the silent truncation' promise: Phase 2 makes expanded upstream context safe by validating prompt size before dispatch.
• The review notes the Phase 1 parser only accepts 'full' | 'summarize'; this issue should finish the planned { maxChars: N } object form and CLI/DAG precedence model instead of leaving authors with a vague rejection.

Acceptance Criteria

• Extend DAG.outputPolicy validation to support upstream: 'full' | 'summarize' | { maxChars: number } plus maxPromptChars.
• Add preflight prompt-length accounting before agent.send(stitched); overflow marks the task BUDGET-EXCEEDED and names parent contribution sizes.
• Add CLI overrides --output-policy-upstream <full|summarize|maxChars:N> and --max-prompt-chars <N> with documented DAG-vs-CLI precedence.
• Reuse the existing EXIT_BUDGET_EXCEEDED = 4 path and preserve downstream skip behavior for budget-exceeded parents.
• Cover default behavior, full late-marker pass-through, overflow messaging, and CLI/DAG precedence with focused tests.

Verification

• pnpm -F @flatbread/proof test
• pnpm -F @flatbread/proof typecheck
• pnpm verify

Related Phase Issues

• Proof output retention: Phase 0/1 follow-up hardening #200 — Proof output retention: Phase 0/1 follow-up hardening: Proof output retention: Phase 0/1 follow-up hardening #200
• Proof output retention Phase 2: upstream prompt policy and budget preflight #201 — Proof output retention Phase 2: upstream prompt policy and budget preflight: Proof output retention Phase 2: upstream prompt policy and budget preflight #201
• Proof output retention Phase 3: resume, supervisor, and disk ergonomics #202 — Proof output retention Phase 3: resume, supervisor, and disk ergonomics: Proof output retention Phase 3: resume, supervisor, and disk ergonomics #202
• Proof output retention Phase 4: oracle evidence alignment #203 — Proof output retention Phase 4: oracle evidence alignment: Proof output retention Phase 4: oracle evidence alignment #203
• Proof output retention Phase 5: documentation and skill refresh #204 — Proof output retention Phase 5: documentation and skill refresh: Proof output retention Phase 5: documentation and skill refresh #204

Artifact Linkage

This issue is linked back from docs/proposals/proof-output-retention-plan.md and, where relevant, docs/proposals/proof-output-retention-review.md so the repo artifacts and GitHub issues remain navigable in both directions.