agent: Add Workflow-Id and Workflow-Step git commit trailers

[krokoko]

Component

Agent (Python runtime)

Describe the feature

Extend the agent's prepare-commit-msg hook so commits carry workflow attribution trailers in addition to the existing Task-Id and Prompt-Version trailers.

Trailer	Example	Purpose
Workflow-Id	coding/new-task-v1	Which workflow executed
Workflow-Step	implement	Active step name when the commit was created

Document the full trailer set in agent/README.md and docs/design/OBSERVABILITY.md.

Use case

Task-Id and Prompt-Version link commits to a task and prompt hash, but not to where in the workflow the commit occurred. When a task fails at verify_build after multiple agent commits, reviewers cannot tell from git log alone which step produced which diff. Attribution and postmortems require cross-referencing TaskEvents or CloudWatch logs.

Proposed solution

1. Set WORKFLOW_ID and WORKFLOW_STEP in the step runner at each step boundary (agent/src/workflow/runner.py); set before agent execution and safety-net commits.
2. Extend agent/prepare-commit-msg.sh to append trailers when set (idempotent, same pattern as Task-Id).
3. Ensure pipeline.py post-hook commits run through the hook or set trailers explicitly.
4. Tests: unit test hook output; integration test that a workflow run produces expected trailers on at least one commit.

Other information

• Builds on existing agent/prepare-commit-msg.sh.
• Related: feat(observability): end-to-end task attribution and cross-plane trace correlation #245 (broader attribution), WORKFLOWS.md.

Acknowledgements

• I may be able to implement this feature
• This might be a breaking change