docs: plan proof output retention

[tonyketcham]

Summary of changes

Plan, judge, implement, and review Proof output retention with one commit per phase. The implementation keeps canvas display output bounded while using full kind: "task" transcripts for execution consumers, convergence parsing/context, sidecars, artifacts, and pinned-artifact resume paths.

Verification completed:

• pnpm -F @flatbread/proof typecheck
• pnpm -F @flatbread/proof build
• pnpm -F @flatbread/proof test
• pnpm verify
• Fresh /proof verification DAG: runtime + contract verification, 2/2 succeeded.

Closes #

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• I added doc comments to any new public exports, and inline comments to any hard-to-understand areas
• My changes generate no new console errors locally
• If applicable, try to include a test that fails without this PR but passes with it

Does this introduce any non-backwards compatible changes?

• Yes
• No

Does this include any user config changes?

• Yes
  • If so, I have updated the relevant areas of documentation
• No