feat: support BYO filesystem (S3 Files / EFS) in schema and CLI

[johanneslanger]

Summary

Adds support for BYO (bring-your-own) filesystem configurations in agentcore.json and the CLI, enabling S3 Files access points and EFS access points alongside the existing session storage.

Closes #1205

Changes

Schema (src/schema/schemas/agent-env.ts)

• Added S3FilesAccessPointSchema with ARN pattern validation for arn:*:s3files:* access points
• Added EfsAccessPointSchema with ARN pattern validation for arn:*:elasticfilesystem:* access points
• Extended FilesystemConfigurationSchema to accept sessionStorage, s3FilesAccessPoint, or efsAccessPoint (at least one required per entry)
• Regenerated schemas/agentcore.schema.v1.json

CLI (src/cli/primitives/AgentPrimitive.tsx)

• Added flags: --s3-files-access-point-arn, --s3-files-mount-path, --efs-access-point-arn, --efs-mount-path
• Added buildFilesystemConfigurations() helper that assembles the array from flat CLI options

Validation (src/cli/commands/add/validate.ts)

• ARN + mount path are required as a pair (error if one is missing)
• ARN format validated via existing isValidArn() utility
• Mount path validated against the same /mnt/<name> pattern as session storage

What's NOT in this PR

• CDK IAM permissions (s3files:ClientMount, etc.) — these belong in @aws/agentcore-cdk and will be a follow-up PR against that package
• agentcore create TUI wizard — BYO filesystems are a non-interactive advanced feature; the TUI wizard continues to offer only session storage

Testing

• 8 new schema tests (valid/invalid ARNs, mixed configs, empty configs)
• 9 new validation tests (pair requirements, ARN format, mount path format)
• All existing tests pass (no regressions)
• TypeScript compiles clean

Example usage

agentcore add agent --name my_agent --type byo --build CodeZip \
  --language Python --framework Strands --model-provider Bedrock \
  --code-location ./app/my_agent/ \
  --s3-files-access-point-arn arn:aws:s3files:eu-central-1:123456789012:access-point/skills-ap \
  --s3-files-mount-path /mnt/skills

Resulting agentcore.json:

{
  "filesystemConfigurations": [
    {
      "s3FilesAccessPoint": {
        "accessPointArn": "arn:aws:s3files:eu-central-1:123456789012:access-point/skills-ap",
        "mountPath": "/mnt/skills"
      }
    }
  ]
}