feat: add built-in MCP server (kuronuri serve --mcp)

[sgtao]

Summary

Integrates a Model Context Protocol (MCP) server
directly into kuronuri, so users can expose PII masking as AI tools without a separate
package.

Changes

New: kuronuri serve --mcp

Starts an MCP server over stdio transport, registering two tools for MCP clients such as
Claude Code and Claude Desktop:

Tool	Description
mask_text	Mask PII in text with configurable language, strategy, and tags
list_ner_tags	Return a Markdown table of NER tags for a given language

Claude Code registration example:

{
  "mcpServers": {
    "kuronuri": {
      "command": "kuronuri",
      "args": ["serve", "--mcp"]
    }
  }
}

Dependency

mcp[cli]>=1.0.0 is added as a regular dependency, so pip install kuronuri gives
you the MCP functionality without any extra steps.

Backward compatibility

The existing kuronuri <INPUT> interface is fully preserved via a default-command
routing group (_DefaultMaskGroup). No changes are required for existing users.

Checklist

• mise run fix passes (ruff + ty)
• mise run test-all passes on Python 3.10–3.14 with 100% coverage
• All code and comments written in English
• License: Apache-2.0 (unchanged)
• pyproject.toml: version bumped 0.2.0 → 0.3.0

Modified files

• src/kuronuri/_mcp.py — new MCP server module (FastMCP-based)
• src/kuronuri/_cli.py — added serve subcommand and _DefaultMaskGroup for backward-compatible routing
• pyproject.toml — added mcp[cli] dependency, bumped version to 0.3.0
• tests/test_mcp.py — new tests for MCP tools
• tests/test_cli.py — added TestServeCommand; existing tests updated for new routing
• README.md / CHANGELOG.md — updated docs

---

(On May 18th, Appended Follows)

Testing MCP in Development

To verify the MCP server works before installing from PyPI, register it directly from
your local branch using Claude Code:

setup and check connect

# Move to any working directory outside the repo
cd /path/to/your/workspace

# Register the local build as an MCP server
claude mcp add kuronuri -- uv run --directory /path/to/kuronuri kuronuri serve --mcp

# Confirm the server is connected
claude mcp list
# Expected output:
# kuronuri: uv run --directory ... - ✓ Connected

Usage

Then launch Claude Code and send a masking request:

claude

Example prompt (Japanese):

「山田太郎　は yamada.taro@example.com のメールアドレスを持ってます」を黒塗りにして下さい

Expected behavior:

• Claude Code calls the mask_text MCP tool automatically
• The masked result is returned inline:

████　は ███████████@███████████ のメールアドレスを持ってます

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