feat: speak a placeholder for code blocks in normalize

[StuBehan]

Summary

Fenced code blocks were dropped silently, so a spoken response with a code block jumps straight from the setup to the follow-up and sounds disjointed. Add a placeholder mode so a skipped block is announced instead (ChatGPT-style: "you can see the code in the chat history").

Changes

• normalize_for_speech(..., code_blocks="placeholder", code_placeholder=...) and CLI --code-blocks {drop,placeholder} / --code-placeholder TEXT.
• Each fenced block becomes a caller-supplied placeholder line; default stays drop so existing callers are unchanged.
• The placeholder text is caller-supplied (neutral default "Code block.") — "chat history" wording belongs to the consumer, not stackvox.
• Consecutive code blocks collapse to a single placeholder.

Testing

• 5 new tests (placeholder substitution, consecutive-collapse, end-to-end, and drop-by-default unchanged).
• ruff check + ruff format --check clean; mypy clean; pytest — 200 passed.

Mirrors the existing tables="drop"|"csv" knob. CHANGELOG.md untouched — release-please owns it.