feat(delegation): persona kwarg for delegate_task

[arminanton]

Adds a persona: str = "" kwarg to delegate_task. When non-empty, the persona body is prepended to the subagent's prompt with an explicit AUTHORITATIVE PERSONA — follow these rules over any default subagent guidance delimiter — a clean replacement for stuffing persona text into the goal/context. Scoped to the persona feature on tools/delegate_tool.py. Draft for review.

[arminanton]

Rebased onto current main and self-assessed against it (no prior review comments on this PR).

Against current main the persona kwarg was only partially wired — it would never have reached the model. Completed the wiring end-to-end:

• _build_child_system_prompt prepends the persona ahead of the goal with an === AUTHORITATIVE PERSONA === delimiter;
• threaded through _build_child_agent → delegate_task (single-task + per-task batch, per-task beats top-level);
• added persona to DELEGATE_TASK_SCHEMA at both the top level and in tasks[].items (otherwise the model can't pass it);
• wired both dispatch paths (run_agent._dispatch_delegate_task + the registry handler).

Added TestChildSystemPromptPersona (4 tests, anti-tautological — asserts the persona text materially precedes the goal in the constructed prompt, and persona=''/whitespace leaves the prompt byte-identical). tests/tools/test_delegate.py → 73 passed. Rebased clean onto current main.