feat: inject $schema URL into generated agentcore.json

[Hweinstock]

Description

Follow-up to #661 (JSON Schema generation). Injects a $schema URL into agentcore.json so IDEs automatically pick up validation and autocomplete.

Three changes:

1. Added $schema to AgentCoreProjectSpecSchema, so it now accepts the field on read, and it's optional for backwards compatibility with existing projects
2. Added SCHEMA_URL constant in config-io.ts pointing to the custom domain of the schema.
3. writeProjectSpec injects $schema via spread before calling validateAndWrite.
4. update the schema with the latest changes.

Also, refactor to avoid duplicating createDefaultProjectSpec in three places.

Notes:

• we DO NOT inject the schema into existing projects.
• small behavior change where remove all on a project maintains our tags. I think this is an improvement in the case that a user starts from fresh and we still want the tags, but wanted to callout.

Remaining Work

• bake into release process automatically.

Related Issue

Closes #

Documentation PR

N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

How have you tested the change?

• I ran npm run typecheck
• I ran npm run lint

End-to-end tested:

1. New project creation injects $schema into agentcore.json
2. Existing projects without $schema still read and deploy correctly
3. Deploy with $schema present — agent + resource created, invoked successfully
4. Deploy without $schema — identical behavior
5. Adding resources via CLI preserves $schema through read-modify-write
6. Manually editing agentcore.json with $schema present deploys without issues

Verified url is live and point to schema on main: https://schema.agentcore.aws.dev/v1/agentcore.json

final-demo.mov

Checklist

• I have read the CONTRIBUTING document
• My changes generate no new warnings
• Any dependent changes have been merged and published

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