docs: add AI Agent Integration section to README

[Alberto-Codes]

Summary

Add a dedicated "AI Agent Integration" section to the README (and PyPI long description) that speaks directly to AI coding agents and developers using them.

Motivation

The README is the first thing both humans and AI agents read when encountering a new tool. Currently it speaks to human developers only. AI agents (Claude Code, Cursor, Cline, Copilot) read READMEs to understand how to use tools. A dedicated section with agent-oriented instructions provides an immediate on-ramp.

Proposed content

For AI agents

• Copy-paste CLAUDE.md / .cursorrules snippet: "After modifying Python functions, run docvet check --files <changed_files> and fix all findings before committing."
• --format json example showing structured output (link to feat: add --format json structured output for agent consumption #151)
• Minimal [tool.docvet] pyproject.toml config
• Quick reference of all subcommands with one-line descriptions

For developers using AI agents

• How docvet fits into the agent coding loop: change → check → fix → commit
• Why docvet catches what agents break (freshness check rationale)
• Setup instructions for popular agent tools

Related

• feat: add docvet init command for agent-ready project scaffolding #150 (docvet init will scaffold these snippets automatically)
• feat: add --format json structured output for agent consumption #151 (--format json for structured agent output)