Add per-run system prompt extension hook

[KirschBluteX]

Summary

• Add a before_agent_start extension hook that chains run-scoped system prompt replacements in registration order.
• Pass the final prompt through a keyword-only AgentHarness run argument so initial, tool-loop, and overflow-retry requests share it while the configured base prompt and transcript stay unchanged.
• Expose frozen, repr-hidden prompt inputs without skill content, and document the hook with a tested packaged example and release note.

Handler exceptions and invalid returns are diagnosed and skipped, so later handlers still run. Static prompt guidelines remain supported and are included in the structured input snapshot. Reload and session replacement refresh that snapshot without persisting run-specific replacements.

Verification

• Linux/Python 3.12: 1533 passed, 3 skipped
• Focused hook, harness, example, and package-metadata tests: 142 passed
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy: no issues in 102 source files
• Hugo 0.152.2: 33 pages built
• Windows full-suite failures are limited to existing path-casing, shell, permission, and TUI platform differences; focused feature tests pass

Closes #532