#71 Markdown: enable GFM tables + task lists + strikethrough (Options::empty() disables them)

markdown_to_html uses Options::empty(), so GFM features are off. Most visible consequences:

1. Tables render as literal pipes. agents.html shows: | field | rule | |------|------| | title | required, plain text, <=300 chars | … Any README/wiki/issue with a table is unreadable.
2. Task lists render as literal [ ] — .forge issues use - [ ] checklists everywhere (see any issue page).
3. No strikethrough, no autolinks (https://… stays plain text), no footnotes.

This is the cheapest perceived-quality win in the batch: three option flags.

Scope

• Options::ENABLE_TABLES | ENABLE_TASKLISTS | ENABLE_STRIKETHROUGH | ENABLE_FOOTNOTES (drop footnotes if it complicates the CSS budget; tables + task lists + strikethrough are the must-haves; add ENABLE_GFM? check the pulldown-cmark 0.13 API).
• Style them: table (border-collapse, --border, --bg-th header like the existing data tables), th/td padding matching the file tables, input[type=checkbox] inline + disabled look (no JS), del in --fg-dim, .footnotes small.
• Task list checkboxes: pulldown-cmark emits <input disabled type="checkbox">; ensure they are disabled (they are) and don't inherit the big form input styling.
• Do the URL scheme filter from #62 in the same pass — same Event/Tag mapping function, one edit.
• Tests: table → <table>; - [x] → checked checkbox; ~~x~~ → <del>; https://x linkified only if autolinks enabled.

Acceptance

• agents.html renders a real table
• task-list items show checkboxes (checked/unchecked)
• blockquote/table inside lists still looks right
• javascript: links blocked (see #62)
• new table CSS stays inside the (raised) CSS budget