Refactor workflow API contracts

[odyslam]

Supersedes the closed oversized PR #90. This is stack slice 5/8.

Base: codex/pcl-output-contract-workflows
Head: codex/pcl-workflow-contracts

What changed:

• Refactors API workflow contracts into clearer definitions.
• Moves workflow request/action metadata toward a contract-oriented model.
• Keeps the schema/manifest surfaces aligned with the workflow commands.

Review focus:

• Workflow/action contract definitions.
• Schema and manifest output compatibility.
• Any behavior change hidden inside the contract migration.

Validation:

• make ci passed on the stack tip codex/pcl-slim-generated-client.
• Local ./target/debug/pcl schema list --toon and api manifest --toon smoke passed.

[odyslam]

Superseded by #100, which now combines workflow contract definitions and workflow module architecture as review step 3/5.