docs: canonicalize GitHub workflow guidance

[codeforester]

Summary

• make docs/github-workflow.md the canonical source for the issue, branch,
  worktree, pull request, and superseded-PR workflow;
• reduce AGENTS.md, CONTRIBUTING.md, skills.md, STANDARDS.md, and
  .ai-context/WORKFLOWS.md to audience-specific summaries and links; and
• update the workflow contract test to enforce canonical-link guidance rather
  than duplicated procedure text.

Issue

Fixes #1949

Validation

• pytest tests/test_contract_hardening.py tests/test_base_cli_docs.py tests/test_docs_version.py -q
• git diff --check
• Result: 16 passed.

Demo Impact

None. This is documentation and contract-test maintenance only.

Notes

No workflow policy changed; the detailed procedure remains in
docs/github-workflow.md.