feat: add DAG convergence config to proof

[tonyketcham]

Summary of changes

• Add an optional top-level converge block to Proof DAG JSON so authors can declare the convergence task and default iteration ceiling in the DAG itself.
• Resolve convergence settings through a shared helper so precedence is CLI override > DAG converge config > historical runner default.
• Add focused @flatbread/proof tests plus README/example updates for the new contract.

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

Testing

• pnpm -F @flatbread/proof typecheck
• pnpm -F @flatbread/proof build
• pnpm -F @flatbread/proof test
• pnpm exec proof --dry-check-cmds --dag .cursor/skills/proof/examples/example_dag.json

Notes

• This branch is stacked on top of toeknee/flatbread-task-dag-instances-18a9 from PR pmf: land content-model docs, typed reads, and proof DAG loops #177.
• The implementation was initially authored through a Proof supervisor run with a self-review task, then tightened with one manual follow-up test expansion and a formatting cleanup after reviewing the generated findings.