Add ESLint plugins for Agent Skills frontmatter validation

[gtbuchanan]

Summary

Adds two new packages that compose to validate SKILL.md files at lint time:

• @gtbuchanan/eslint-plugin-md-frontmatter — generic. One rule, md-frontmatter/schema, validates Markdown YAML frontmatter against any JSON Schema via ajv. Each error is mapped back to the offending YAML node range for inline reporting.
• @gtbuchanan/eslint-plugin-agent-skills — focused. Ships:
  • skillFrontmatterSchema — the Agent Skills JSON Schema as a real .json file with $id set to a future canonical URL.
  • agent-skills/name-matches-dir — the one spec constraint pure schemas can't express.
  • configs.recommended — a ready-to-spread flat-config block that wires both plugins plus core max-lines: 500 for **/skills/*/SKILL.md. One-line setup for consumers: [...configs.recommended].

eslint-config delegates to configs.recommended. ajv ^8.20.0 added to the workspace catalog. resolveJsonModule: true moved to the shared @gtbuchanan/tsconfig/node.json so consumers get JSON imports by default.

Plugin-level RuleTester unit tests (test/*.test.ts per package) cover rule behavior. End-to-end coverage in eslint-config/e2e/skill.test.ts exercises the fully-wired pipeline against synthetic SKILL.md fixtures.

CHANGELOG.md stubs added to both new packages matching the existing format. No changeset — these will land as initial 0.1.0 entries on first release.

Relates to #41 — sets up the lint-side validation that #41 calls out as a CI checklist item. Follow-up rules tracked in #68.

Test plan

• pnpm install && pnpm check — 32/32 turbo tasks green
• pnpm test:e2e — 24 e2e tests pass (18 existing + 6 new SKILL.md cases)
• Rules fire on a synthetic bad SKILL.md (uppercase name, missing description, unknown extra field, mismatched parent dir):

  mkdir -p packages/cli/skills/test-bad
  printf -- '---\nname: WrongName\nunknown: foo\n---\n# Body\n' \
    > packages/cli/skills/test-bad/SKILL.md
  pnpm -C packages/cli exec eslint --no-cache skills/test-bad/SKILL.md
  rm -rf packages/cli/skills/test-bad

  Confirmed: 4 plugin warnings + 1 Prettier warning at 2:1 (missing description), 2:7 (name pattern + dir mismatch), 3:1 (unknown field), 5:1 (no trailing newline).
• Existing packages/cli/skills/gtb-build-pipeline/SKILL.md lints clean
• src/schema.json is shipped in the eslint-plugin-agent-skills tarball (verified via tar -tzf)

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