Update contributor docs: CoC 2.1, completeness, link fixes

- Upgrade CODE_OF_CONDUCT.md from Contributor Covenant 1.4 to 2.1 (tiered enforcement ladder, expanded pledge)
- Add project conventions section to CONTRIBUTING.md (line length, Ruff rules, MyPy, rank_zero_*, callback hook convention)
- Add two-package architecture overview and Python 3.9+ requirement to CONTRIBUTING.md
- Fix broken/dead links in CONTRIBUTING.md and PULL_REQUEST_TEMPLATE.md
- Rewrite tests/README.md: test layout tree, RunIf decorator, markers, standalone tests, coverage commands for both packages

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Co-authored-by: Claude Code <[email protected]>