Support .md skill files as user-configurable default skills

[lukemelia]

What

Lets a user's active System Card supply the skills enabled by default in new AI assistant rooms, referencing the new .md skill files (MarkdownDef whose boxel.kind: skill frontmatter makes them a skill source) rather than only legacy Skill cards.

Previously the default LLM model for new rooms came from the user's system card, but default skills were a hardcoded list in the host pointing at Skill/*.json cards — not user-configurable and card-only.

Changes

• SystemCard — new defaultSkills = linksToMany(MarkdownDef) field + a "Default Skills" section in the isolated view.
• loadDefaultSkills(submode) now returns skill ids: the system card's defaultSkills when set (applied to every new room, mode-agnostic), otherwise the hardcoded fallback — which is repointed from legacy Skill/* cards to skills/*/SKILL.md.
• Room creation (fork A) — CreateAIAssistantRoomInput gains enabledSkillIds/disabledSkillIds; create-ai-assistant-room resolves ids kind-agnostically via loadSkillSource (splitting .md file uploads vs Skill card uploads, gathering commands via getSkillSourceCommands) and bakes them into the room's initial state.
• "Add same skills" is now id-based, so .md skills survive that flow (previously dropped by an isCardInstance filter).
• Seed — the default SystemCard instance (catalog realm) is populated with the three default .md skills via universal @cardstack/skills/... refs. (Committed separately in the catalog contents repo.)

When a user has no system card, or it lists no skills, new rooms fall back to the (now .md) hardcoded defaults — no regression out of the box.

Verification

• ✅ glint type-check (ember-tsc --noEmit) clean across host app + tests.
• ✅ eslint clean on changed host files. Base .gts "parse errors" under bare/lint-staged eslint are a known content-tag-preprocessor artifact (reproduce on untouched .gts); the glint path is clean.
• ⏳ Host acceptance + matrix suites need CI — the default-skill id change ripples through several skill tests (they import the repointed constants; the live test realm-server serves the .md skills).

Notes / follow-ups

• Bundled system-card.d.ts is a gitignored build artifact, regenerated from the source change.
• The catalog default.json seed lives in the nested catalog contents repo and needs its own PR/commit.
• Worth confirming in a running env: cross-realm universal-ref resolution for the seeded defaultSkills, and that linksToMany defaultSkills ids are available on the loaded system card at room-creation time.

🤖 Generated with Claude Code

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

Host Test Results

    1 files      1 suites   2h 15m 5s ⏱️
3 269 tests 3 227 ✅ 15 💤  1 ❌ 26 🔥
3 287 runs  3 219 ✅ 15 💤 27 ❌ 26 🔥

Results for commit 4635503.

For more details on these errors, see this check.

Realm Server Test Results

    1 files      1 suites   9m 50s ⏱️
1 674 tests 1 674 ✅ 0 💤 0 ❌
1 753 runs  1 753 ✅ 0 💤 0 ❌

Results for commit 4635503.