docs: surface dev install, lint, typecheck, and test commands in README

[dharaneeshexe-web]

What this PR does / why we need it:
Closes #966

Adds the following to the ## Development section in README.md:

• Dev dependency install command (pip install -e ".[dev]")
• Manual lint, typecheck, and test commands surfaced from CLAUDE.md
• Pull request guidelines (Conventional Commits title format)

Special notes for your reviewers:

• The information already existed in CLAUDE.md and pyproject.toml but was not discoverable by human contributors.
• No code changes, documentation only.