Add server-specific heuristics to prevent spawning servers in irrelevant projects

[bug-ops]

Summary

Add minimal heuristics to prevent spawning LSP servers in projects where they're not applicable. This prevents the MCP server from exposing useless tools.

Context

Related discussion: #34 (comment)

When servers spawn outside their relevant project contexts (e.g., rust-analyzer in a Python project), they create noise and expose tools that can't be used.

Proposed Solution

Implement server-specific heuristics in spawn_batch() to check for project markers before spawning:

fn should_spawn_server(name: &str, config: &ServerConfig) -> bool {
    match name {
        "rust-analyzer" => Path::new("Cargo.toml").exists(),
        "pyright" => Path::new("pyproject.toml").exists() || Path::new("setup.py").exists(),
        _ => true // unknown servers get a chance
    }
}

Implementation Considerations

• Check markers from workspace root
• Graceful fallback for unknown servers (allow them)
• Document which servers have heuristics
• Add tests for heuristic logic

Related Issues

• Fixes Don't require rust-analyzer #32 (graceful degradation)
• Related to Phase 5 of PR Issue #32: Implement graceful LSP server degradation #34