fix: discover tilde fences and Taskfile mappings

[rogerchappel]

Summary

• recognize CommonMark backtick and tilde code fences while requiring matching closing fence characters and lengths
• normalize scalar and inline mapping-form Taskfile cmds entries to executable command text
• add fixture-backed risk, tool, location, fence-language, and boundary regressions and document supported forms

Commit-by-commit:

• 36b283d test: cover fenced docs and Taskfile mappings adds the failing publish-discovery fixtures and assertions.
• 594a40d fix: discover supported fenced and Taskfile commands implements matching Markdown fence parsing and Taskfile mapping normalization.
• 5a94f35 docs: describe executable discovery forms documents supported fences and Taskfile command entries.
• 11b4f83 test: verify matching fence boundaries and languages covers mismatched fence markers and non-shell languages.

Verification

• Tests or checks run: npm run release:check; git diff --check
• Manual review completed: confirmed npm publish is reported as dangerous from both docs/runbook.md and Taskfile.yml, with Taskfile tools equal to ["npm"].

Risk Level

• Low
• Medium
• High

Notes: The Markdown parser now tracks fence character and minimum closing length; unsupported languages remain excluded.

Rollback Plan

• Revert the four commits in this pull request.

Human Decision Needed

• None
• Maintainer review
• Product/design decision
• Security/privacy review
• Other: