feat: add natural chat style to root prompt

[QueryPlanner]

What

Add a natural-chat style layer to the root Blacki prompt.

Why

Make replies feel more conversational and context-aware while keeping serious requests precise and bounded.

How

• Add guidance for pace, message shape, reaction tokens, emphasis, emoji, callbacks, and register shifts.
• Include concrete examples such as "you actually DID that?", "fantastic 🫠", "very responsible 🫡", "coffee situation again?", and "sjfksjfks".
• Cover the reference's examples for stretched words, punctuation, timing, stage directions, code-switching, style matching, open-ended jokes, keysmashes, and informal typos.
• Mirror user language gradually without forcing slang, typos, keysmashes, or code-switching.
• Keep safety, privacy, accuracy, explicit formatting, and tool authorization ahead of style.
• Preserve the concise delegated-worker prompt and add focused contract tests.

Tests

• uv run ruff format --check
• uv run ruff check --output-format=github
• uv run mypy .
• uv run pytest --cov=src --cov-report=xml --cov-report=term-missing
• Pre-commit hooks: Ruff and MyPy

Related Issues

No related issue.