[Refactor] Move active portable schemas to `/.schemas`

[ashleyshaw]

Is your code refactoring request related to a problem?

Portable schemas currently live under .github/schemas, which blurs the line between GitHub-native repo configuration and reusable plugin validation.

Parent epic: #283.

Describe the Refactoring / Review Task

Move only active, portable schemas into /.schemas and update schema references in docs, VS Code settings, and validators.

Use Case

Plugin, skill, agent, hook, workflow, and frontmatter schemas should be usable outside GitHub's special .github folder.

Acceptance Criteria

• Active schemas are identified.
• Portable schemas move to /.schemas.
• Repo-only schema references remain valid.
• VS Code schema mappings are updated.
• Validators reference the new path.
• PR uses correct branch prefix refactor/.

[linear-code]

LS-612

[ashleyshaw]

Completed and verified locally.

Evidence:

• Portable schemas migration report: .github/projects/active/portable-ai-plugin-restructure/portable-ai-plugin-restructure-portable-schemas-migration-report-2026-05-20.md
• Portable schema source folder: /.schemas
• Validators and package scripts point at .schemas/**/*.json.
• Migration map rows are tagged with [Refactor] Move active portable schemas to /.schemas #297 for moved/converted schema assets.

Verification run on 2026-05-26:

• npm run validate:json:schemas passed: 5 files, 5 syntax-valid, 5 schema-valid, 0 invalid.
• npm run validate:agents passed against the moved frontmatter schema: 13 passed, 0 failed, 6 skipped.
• npm run validate:structure passed.
• npx markdownlint-cli2 .schemas/README.md .github/schemas/README.md .github/projects/active/portable-ai-plugin-restructure/portable-ai-plugin-restructure-portable-schemas-migration-report-2026-05-20.md passed with 0 errors.
• git diff --check passed.

Also removed committed conflict markers from .schemas/README.md before closing this issue.