Make package README links portable

[fly1d]

Why

README.md is the package long description. Its repository-relative links work on GitHub but would point to nonexistent paths when rendered on PyPI. The examples guide also still linked to the removed #install heading.

Reviewer decision: confirm that the package page should link to the canonical documentation on main while releases remain pre-alpha.

What changed

• Use canonical GitHub URLs for every README documentation and policy link.
• Point the examples guide to the current first-run section.
• Add a packaging contract that rejects relative inline and reference-style Markdown link targets.
• Record the fix in the changelog.

This does not publish a package, change runtime behavior, or modify an existing release.

Verification

• python -m pytest -q: 762 passed, 4 skipped; 765 subtests passed.
• python benchmarks/run_offline.py: 16 passed.
• python scripts/check_docs.py: 48 Markdown files passed.
• python scripts/check_quickstart.py: demo, Doctor, reduction, validation, and 2/2 replay passed.
• python -m twine check <wheel> <sdist>: both passed.
• python scripts/check_release_artifacts.py --tag v0.1.0.dev11 <wheel> <sdist>: valid.

Disclosure

• No secrets, credentials, private URLs, proprietary source, or confidential logs are included.
• Documentation and changelog entries are updated.
• Material LLM, agent, or automation involvement: OpenAI Codex prepared and tested this change; an independent agent review identified the reference-style-link regression case.