Remove API key requirements and add public MCP plugin

[justincasher]

Summary

• make the Python API client, CLI search, and stdio MCP API backend work without credentials
• continue accepting legacy API-key arguments and environment variables while ignoring them
• add repository plugins for Claude Code and Codex that connect to the public hosted MCP endpoint
• constrain the MCP SDK to mcp>=1.28.0,<2, document anonymous per-IP limits, and report CLI network failures cleanly
• update OpenAPI and user documentation for anonymous access

The module_data.json change is regenerated documentation output associated with the package documentation updates.

Validation

• 140 API client, CLI, and MCP tests pass
• Ruff checks and formatting pass for the changed Python files
• Codex plugin validation passes
• Claude and Codex plugin JSON plus OpenAPI YAML parse successfully

Required rollout order

1. Merge this client/package PR (Remove API key requirements and add public MCP plugin #82) so the compatible MCP dependency is available on main.
2. Merge and deploy the companion production app PR (Refactor package_config into separate modules #47). Its deploy script resets the sibling lean-explore checkout to origin/main.
3. After the public server endpoint is live, publish the client package to PyPI.
4. Announce the Claude Code and Codex plugin.

Do not publish the client before step 2: this PR intentionally sends no API credentials, while the old production server still returns 401. Do not deploy #47 before step 1: the hosted ASGI MCP app needs the MCP SDK constraint introduced here.

Companion production app PR: https://github.com/justincasher/lean-explore-app/pull/47