Generate lab page blocks as markdown instead of Liquid includes

[ivorb]

Recap of the problem

The lab instruction pages are read directly by platforms that never run Liquid. GitHub's own markdown renderer is one — passing a page through the GitHub markdown API returns:

<p>{% include lab-tasks-table.html lab='A' %}</p>

So anyone browsing a lab on github.com sees the raw tag where the task table should be.

No lab page on main has ever contained Liquid — only index.md. This restores an existing convention rather than inventing one.

How it works

tools/generate_lab_blocks.py writes both generated blocks as real markdown between HTML comment markers, which are invisible in every renderer:

<!-- BEGIN GENERATED: task-table - do not edit by hand; run: python tools/generate_lab_blocks.py -->
| Section | Task | Level | Time |
| **Core** | [Task 1 – Create and ground an agent](A1-create-and-ground-an-agent.md) | ▰▰▱▱▱ L200 | ~15 min |
...
<!-- END GENERATED: task-table -->

Two blocks are generated: the task table on each landing page, and the access notice on each gated task.

Frontmatter stays the single source of truth. --check runs in content-checks and makes drift a build failure — the same pattern as Labfiles/_shared/sync.py --check. A lab page missing its markers is a hard error, so a new lab can't silently ship without its table.

_includes/ is removed entirely. The web-only pages — workshop.md, explore.md, labs.json — keep Liquid, because they're never consumed as raw markdown.

Two problems this fixes beyond portability

Link checking is back to 117 references, from 103. The 14 task links became literal markdown again, so check_links.py can see them. That regression was introduced when the tables became generated.

The cross-branch dependency disappears. Lab D's landing page referenced an include that existed only on the PoC branch — it would have failed the site build, and is why #234 is currently a draft. With generated markdown there's nothing to depend on.

Verification

• Generated output matches what the Liquid produced, including the totals — lab A still reports 35 minutes core and 2 hours 25 minutes full
• The generator is idempotent: a second run reports 0 files updated
• Negative test: corrupting a block makes --check exit 1 with a diff, and re-running the generator restores it
• All Tier 0 checks pass; the three web views still render

Stacking

Branches off gated-task-notices (#233), because it converts both includes — the table from #231 and the notice from #233. Merge order: #231 → #233 → this.

If you'd rather not merge an "add include, then replace it" sequence, I can fold this back into #231 and #233 so they ship pre-rendered from the start. Say the word — it's mechanical, but it means force-pushing two PRs you may already have reviewed.

[ivorb]

Folded into #231 and #233 - both now ship pre-rendered markdown from the start, rather than adding a Liquid include and replacing it in a follow-up. Same final content, verified byte-identical to this branch.