docs: add `--read-only` to the MCP setup examples in README

[Kayforkind]

Problem

The README documents the MCP server's --read-only flag in the security paragraph, but the copy-paste setup examples (claude mcp add and the mcp.json snippet for Cursor/Cline) don't show it. New users wire up the full 10-tool server when 3 read tools would do.

Scope

• In the README's "Agentic usage" section, update the claude mcp add example and the mcp.json example to pass --read-only as the recommended default.
• Add one sentence explaining what it does (registers only the 3 read tools: pdf_info, pdf_extract_text, pdf_search_text) and how to drop the flag for full access.

Acceptance criteria

• Both setup examples show --read-only.
• One-line explanation of the flag sits next to the examples.
• No other README content changes (keep the diff focused).