feat: expose and control system prompt from admin UI

[mattmezza]

Summary

• Add a new system prompt controls panel in the LLM admin tab with live preview, section breakdown, copy actions, and runtime prompt capture browsing.
• Add backend debug endpoints for assembled prompt preview and recent captured runtime prompts, plus config-driven override support for tool usage/history handling blocks.
• Refactor prompt composition into a shared core/prompt_builder.py used by runtime and admin preview, and add tests for the new admin API routes.

Verification

• uv run ruff check .
• uv run pytest tests/test_admin_ui.py tests/test_goal_decomposition.py tests/test_task_reflection.py tests/test_memory_extraction.py tests/test_memory_consolidation.py

Notes

• Prompt capture is opt-in via admin.capture_prompts and stores only a small in-memory ring buffer (not persisted).
• Captured prompt metadata hashes user/chat identifiers before exposing them in admin debug output.