feat: annotate workshop pages with journey/adventure frontmatter for learner-profile filtering

[Copilot]

Workshop pages had no metadata for filtering by learner type or content category, making it impossible to programmatically surface the right pages for a given user profile.

Changes

Frontmatter schema — two new fields on every workshop page

---
journey: all        # learner path: all | ui | terminal | codespace | local | copilot
adventure: core     # content category: core | setup | scenario-a | scenario-b | scenario-c | scenario-d | advanced | side-quest
---

• journey maps to the learner's environment/path (e.g. ui = browser-only, terminal = any shell, codespace/local = environment-specific subsets, copilot = Copilot app/Agents tab)
• adventure maps to workshop structure (core shared path, setup, one of four scenario branches, post-core advanced topics, or optional side-quest deep dives)

107 workshop files annotated

All workshop/*.md files (excluding README.md) — core steps, branch variants (NNa/b/c), and side quests — prepended with the appropriate frontmatter block derived from filename pattern and content scope.

Schema documented in guidelines.md

New "Workshop page frontmatter schema" section covers valid values, placement rules, assignment logic by filename pattern, and side-quest journey edge cases.