docs: fix README dev-deps command and stale .github template placeholders

[w7-mgfcode]

GitHub-presence audit found P0 onboarding/template defects:

• README.md:42 — uv sync is missing --extra dev; a fresh clone following the README gets no dev dependencies (ruff/mypy/pytest). CLAUDE.md's authoritative command is uv sync --extra dev.
• .github/ISSUE_TEMPLATE/config.yml:4 — Documentation contact link points at the your-org placeholder; dead link served to every new-issue author. Should be w7-mgfcode.
• .github/CODEOWNERS — @your-username placeholder x8; CODEOWNERS is non-functional (no auto review-requests fire). Should be @w7-mgfcode.

[w7-mgfcode]

Resolved — merged into dev (PR #141 / #143). Auto-close did not fire because the PRs targeted dev, not the default branch.