chore: migrate type checking from mypy to pyright

[hey-granth]

Summary

mypy is slow and has worse error messages compared to modern alternatives.
This issue tracks migrating type checking from mypy to pyright, which runs
as a daemon for near-instant incremental checks and is production-stable.

Motivation

• mypy is significantly slower on incremental checks compared to pyright
• Pyright error messages are more precise and actionable
• Pyright is actively maintained by Microsoft and backs Pylance in VS Code
• Daemon mode means subsequent checks in dev workflow are near-instant

Migration Steps

• Add pyright as a dev dependency via uv add --dev pyright
• Add pyrightconfig.json at the repo root
• Run pyright in standard mode and fix all reported errors
• Remove mypy from dev dependencies and pyproject.toml
• Update CI workflow to replace mypy invocation with pyright
• Update CONTRIBUTING.md or dev setup docs if mypy is mentioned

Initial Configuration

{
  "pythonVersion": "3.11",
  "typeCheckingMode": "standard",
  "venvPath": ".",
  "venv": ".venv"
}

Start with standard mode. Strict mode can be a follow-up once the
baseline is clean — strict on tree-sitter and rustworkx bindings may
produce noise from incomplete upstream stubs.

Notes

• Pyright and mypy occasionally disagree on correctness. Pyright is generally correct when they diverge but some annotations may need adjustment during migration.
• Do not migrate to ty (Astral) or Pyrefly (Meta) at this time. Both are pre-release and not production-stable. Revisit when either hits 1.0.

Out of Scope

• Enabling strict mode (follow-up issue if desired)
• Adding inline type annotations to currently untyped functions