[Feature Request] Support configuring a custom IAM permissions boundary for generated roles

[zhaojiew10]

Description

Please add first-class support for attaching a custom IAM permissions boundary to every IAM role that AgentCore generates during agentcore deploy.

Many enterprise and partner AWS accounts enforce an organization-managed permissions boundary on all IAM principals. A common form of this guardrail is an explicit Deny on iam:CreateRole unless the new role attaches a specific policy as its own permissions boundary.

In such accounts, agentcore deploy fails at role creation. Example error when deploying a project with a single harness:

User: arn:aws:sts::<acct>:assumed-role/cdk-hnb659fds-cfn-exec-role-<acct>-<region>/AWSCloudFormation
is not authorized to perform: iam:CreateRole
on resource: arn:aws:iam::<acct>:role/<project>_<harness>
with an explicit deny in a permissions boundary: arn:aws:iam::<acct>:policy/<OrgBoundaryPolicy>
(Status Code: 403; HandlerErrorCode: UnauthorizedTaggingOperation)

The generated roles have no PermissionsBoundary property, so CloudFormation's iam:CreateRole call is denied and the whole stack rolls back. There is currently no supported configuration (CLI flag, aws-targets.json field, env var, or agentcore.json field) to inject a boundary.

I confirmed the root cause is the "must attach boundary on create" rule (creating a probe role with the boundary succeeds; without it 403s). I patched the generated bin/cdk.ts to apply an App-wide boundary via the CDK IAM aspect:

import * as iam from 'aws-cdk-lib/aws-iam';

iam.PermissionsBoundary.of(app).apply(
  iam.ManagedPolicy.fromManagedPolicyArn(
    app,
    'OrgPermissionsBoundary',
    'arn:aws:iam::<acct>:policy/<OrgBoundaryPolicy>'
  )
);

This makes the deploy succeed (stack reaches CREATE_COMPLETE), but it is not viable long-term because agentcore deploy re-materializes the cdk/ directory from CLI templates and overwrites the patch. I must deploy via raw cdk deploy instead, which leaves AgentCore's local state (.cli/deployed-state.json) out of sync.

Acceptance Criteria

• When configured, every generated IAM role (harness/runtime execution roles, and any L3-created roles) is synthesized with the specified PermissionsBoundary.
• Works through the standard agentcore deploy flow with no manual edits to generated cdk/ files.
• Survives cdk/ regeneration/materialization.
• No boundary is applied when the setting is absent (backward compatible).

Additional Context

• @aws/agentcore CLI: 0.28.1
• CDK (bundled): 2.1126.0
• Node.js: 22.x
• Deploy target: single-harness project, PYTHON runtime, region <region>