docs: add open source community standards

[belaytzev]

What

Implements the opensource.guide recommended community documentation:

• CONTRIBUTING.md — dev setup (uv, Python 3.14+), test/lint/format commands, code style (black 24.10.0 pin, E704 Protocol stub gotcha, MD031), Conventional Commits, PR process, bug report guidance
• CODE_OF_CONDUCT.md — canonical Contributor Covenant 2.1, enforcement contact via GitHub
• SECURITY.md — private vulnerability reporting via GitHub Security Advisories, scope notes for credentials/session data
• .github/ISSUE_TEMPLATE/ — structured bug report and feature request forms + config routing security reports to private advisories
• .github/PULL_REQUEST_TEMPLATE.md — checklist mirroring the CI verification suite
• README.md — badges (CI, MIT, Python 3.14+, contributions welcome) and new Contributing/License sections

Why

Repo had LICENSE + README but none of the other community health files GitHub surfaces to contributors. These set contribution expectations and route security reports away from public issues.

Notes

• Code of Conduct text downloaded from contributor-covenant.org, not paraphrased
• No code changes; pre-commit hooks pass on all new files

🤖 Generated with Claude Code