Skill-as-markdown: polymorphic frontmatter field on MarkdownDef

[lukemelia]

Implements the realm representation for skills as markdown (CS-11545): a skill is not a distinct file type. Every .md stays a MarkdownDef; skill-ness is a polymorphic frontmatter field, not a SkillDef subclass.

Design + rationale: Linear CS-11545. De-risking spike: CS-11568.

What this does

• MarkdownDef gains @field frontmatter = contains(FrontmatterField).
• extractAttributes parses YAML frontmatter; a kind → FieldDef registry (kept above MarkdownDef) picks the concrete subclass — SkillField (name/description/commands) for kind: skill.
• The concrete subclass is recorded via meta.fields.frontmatter.adoptsFrom and rehydrates as that type on read (field-level polymorphism).
• Flat searchable kind/description are written to search_doc → searchFiles({ filter: { eq: { kind: 'skill' } } }). (name isn't surfaced flat — it collides with the FileDef filename; it lives in frontmatter.name.)
• Boxel discovers skills by frontmatter kind; Claude Code discovers them by the SKILL.md filename — independent, so one byte-for-byte file serves both.

Closes the spike's write-path gap

The FileDef write path was flat (buildFileResource emitted only top-level meta.adoptsFrom). extractAttributes now routes per-field meta via a global symbol; the file extractor lifts it into the resource's meta.fields (kept out of the flat search_doc) and buildFileResource threads it through. Additive — FileDefs that emit no field meta are unchanged.

Notable

• CommandField extracted into its own module so SkillField reuses it without depending on the legacy Skill card (skill.gts re-exports for back-compat).
• Adds yaml to @cardstack/base.

Validation

Draft: opened to let CI run the build / type-check / tests — these were not runnable locally (needs the full host test-services stack). The half the spike couldn't exercise live — whether meta.fields survives the index storage→read bridge — is what the round-trip test (markdown-skill-frontmatter-test.gts) is here to confirm under CI.

Not in this PR (remaining Phase B)

• Host command-definition upload reading markdownDef.frontmatter.commands (spec §6).
• migrate-skill command + chooser/search surfacing.

🤖 Generated with Claude Code

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

Host Test Results

    1 files      1 suites   1h 34m 55s ⏱️
3 065 tests 2 976 ✅ 15 💤  1 ❌ 73 🔥
3 084 runs  2 922 ✅ 15 💤 74 ❌ 73 🔥

Results for commit e0c8611.

For more details on these errors, see this check.

Realm Server Test Results

    1 files      1 suites   10m 8s ⏱️
1 716 tests 1 709 ✅ 0 💤 7 ❌
1 809 runs  1 802 ✅ 0 💤 7 ❌

Results for commit e0c8611.

For more details on these errors, see this check.