feat(engine): carry rule rationale on every check note

[benjaminjackson]

Summary

Follow-up from #6: check gave an agent a one-line message and nothing else — to learn why a flag was a tell (and decide whether an info-severity one was worth acting on) it had to shell out to explain separately. That's the whole value of the info tier, so it shouldn't be a second round trip.

• Note gains a rationale field, populated from the owning rule.
• JSON output carries it plainly.
• Human output gets a why: line between excerpt and fix, mirroring explain's Why/Fix order.
• README and docs/SPEC.md examples updated to match, including the CLI help banner's note-schema summary.

Example

$ printf 'That is exactly the point.\n' | sloplint check -
-:1:9: warning exactly-the  "exactly the point/kind/problem/…" is an overused LLM emphasis tic.
    excerpt: That is [exactly the point].
    why: 'exactly the X' is a model intensifier that a careful writer rarely leans on.
    fix: Drop 'exactly the'; state the point without the intensifier.

Testing

rspec — 226 examples, 0 failures.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KKRi7mTZKtDeo47FnYMP3t