Feat/run once

[maxim-uvarov]

Summary

• Add run-once fence option that executes a code block once, then rewrites the fence to no-run so subsequent numd run calls skip it. Closes add option run-once #71
• Works with combined options (e.g., nu run-once, try → nu no-run, try)
• Drop short-form 1 — it required a fragile \b1\b regex on the raw fence string, and run-once is inherently a one-time annotation where a short form adds no
  convenience
• Docs: fix stale section title, heading structure, add extract-block-index and merge-markdown pipeline steps to commands explanations
• Fix @example list literal syntax in extract-fence-options

Test plan

• Unit tests for decorate-original-code-blocks verifying run-once → no-run rewriting (alone, combined with other options, plain blocks unchanged)
• Unit tests for classify-block-action and extract-fence-options with run-once combined options
• Integration test file z_examples/6_edge_cases/run_once.md — both run-once and run-once, no-output
• Verified second run produces no output (both blocks become no-run)
• All existing unit tests pass