feat: implement webfetch library, CLI, MCP server, and Python bindings

[chaliy]

Summary

• Implements the webfetch specification with library, CLI, MCP server, and Python bindings
• Full test coverage with 29 unit tests and 19 integration tests
• CI pipeline with check, fmt, clippy, test, build, and docs

What

Implements the webfetch spec (specs/initial.md) including:

Library (webfetch crate):

• WebFetchRequest/WebFetchResponse types with JSON schema support
• HTTP client with async fetch, timeout handling, and partial content
• HTML to markdown/text conversion with entity decoding
• Tool builder pattern with URL allow/block lists
• Comprehensive unit tests

CLI (webfetch-cli crate):

• Binary name: webfetch
• Flags: --url, --method, --as-markdown, --as-text, --user-agent, --llmtxt

MCP Server (webfetch-mcp crate):

• JSON-RPC 2.0 over stdio
• Exposes webfetch tool with proper schemas

Python Bindings (webfetch-python crate):

• PyO3-based bindings
• WebFetchRequest, WebFetchResponse, WebFetchTool classes
• Convenience fetch() function

CI:

• GitHub Actions workflow for check, fmt, clippy, test, build, and docs

Why

Implements the spec defined in specs/initial.md to provide a reusable webfetch tool.

How

• Rust workspace with multiple crates
• reqwest for HTTP client
• Built-in HTML conversion (no external dependencies)
• PyO3 for Python bindings
• MCP protocol over stdio

Risk

• Low - This is a new implementation following the spec
• All tests pass locally

Checklist

• Unit tests are passed
• Integration tests (smoke tests) are passed
• Documentation is in code (llmtxt, docstrings)
• Specs are up to date and not in conflict
• Linting passes (cargo clippy)
• Formatting passes (cargo fmt)

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