feat(contracts): submit C-2 workflow contract (forge-invariant)

[pentaxis93]

Summary

• Adds the source C-2 submit workflow contract producing change-proposal.
• Updates submit manifest wiring from patch to change-proposal and registers revise.
• Rewrites the submit protocol doc as WHAT-layer prose while preserving the release metadata invariant.

Changes

• Added workflow-contracts/submit.toml with initial and revision proposal paths.
• Updated conformance-facing tests for submit's contract, manifest interface, and retained protocol doc.
• Added a changelog entry for feat(contracts): submit C-2 workflow contract (forge-invariant) #330.

GitHub Issue(s)

Closes #330
Refs #318

Test plan

• python -m tooling.conformance
• python scripts/release-check metadata
• python -m unittest discover -s tests

[pentaxis93]

Closing without merge. #330 and #332 were not self-contained and have been merged into a single integrated issue, #340 (submit→review→land disposition migration). The work on this branch is sound and serves as reference for #340, which will be re-planned and delivered as one coherent topology change. No work is lost; nothing from this branch reached main.