Update audited ADR dependencies and remove unmaintained Markdown packages

[Copilot]

PNPM audit reported high-risk paths through sharp, markdown-toc, and related transitive dependencies. This updates supported dependencies and removes unmaintained packages without patched releases.

• Dependency updates

  • Upgrade sharp to ^0.32.6.
  • Upgrade walk-sync to ^2.2.0.
  • Remove unused @lisnote/image-generator.

• Remove unpatched Markdown dependencies

  • Remove markdown and markdown-toc.
  • Use the existing remarkable dependency for Markdown parsing, status extraction, and ToC generation.

• Safe heading anchors

  • Generate deterministic, URI-encoded heading IDs and ToC links.
  • Preserve duplicate-heading handling and add coverage for generated ToC anchors.

return `${indent}${bullet} [${heading.content}](#${encodeURIComponent(heading.slug)})`