docs: align release documentation and verify Markdown examples

[strider2038]

Documentation examples had drifted from the public API: translation snippets used removed methods, the README listed a violation that cannot occur, and group selection and custom violation translation were described incorrectly. Correct those examples and contracts across the guides, godoc, and AGENTS.md.

Shorten the README, move five detailed constraint guides into docs/, add a constraint catalog, and update installation, contribution, and release instructions. Add local Markdown link/heading checks and execution of complete Go examples to the local gate and shared CI. The checker currently covers 22 Markdown files and three complete examples; partial snippets link to executable examples where applicable. Library runtime behavior is unchanged.

Validation on Go 1.27.1:

• bash scripts/test-all.sh: documentation checks and five checker regression tests, gofmt, vet, golangci-lint v2.13.2, unit/race tests, module tidy and verification passed.
• Release tooling regressions: 13 shell checks, 17 release-candidate tests, and four security-report tests passed.
• Verified all catalog API symbols against package sources; git diff --check passed.