feat(prompt): add configurable Mecatl commit co-author guidance

Problem

The harness currently does not give its agents a canonical commit co-author identity. As a result, PR #1591 was committed with Co-authored-by: OpenAI Codex <codex@openai.com> even though Mecatl, not Codex, performed the work.

Desired behavior

By default, the standard model-visible commit guidance instructs every agent that creates a commit to append exactly:

Co-authored-by: Mecatl <noreply@mecatl.dev>

The instruction must reach every applicable agent path through the shared standard/system-prompt assembly, rather than relying on a caller or a skill to remember it.

Configuration

Introduce an operator-tier settings.yaml system-prompt configuration section with an explicit opt-out for this guidance. Proposed shape:

system_prompt:
  commit_coauthor: false

Default is enabled when the key is absent. Project-tier configuration must not be able to disable provenance guidance.

Acceptance criteria

• The default built standard prompt contains the exact Mecatl co-author instruction.
• With the operator opt-out, that instruction is absent.
• Main and child/delegated agents receive the same default guidance through the real composition path.
• The new config parses strictly, appears in generated configuration reference/skeleton output, and documents its operator-only/default-on semantics.
• Prompt-level tests assert the instruction at the layer that owns it, so a duplicate phrase elsewhere cannot mask a missing wiring path.

Non-goals

• Do not rewrite existing Git history or alter externally authored commit trailers.
• Do not add a second, ad hoc commit-message construction path.