fix(deploy): synthesize only selected deployment targets

[proton0210]

Description

agentcore deploy --target dev can fail with Credential "api-key" not found in deployed state because the generated CDK app constructs every configured target, including targets whose credentials have not been provisioned.

Pass the selected target names to the CDK child process and filter before stack construction. CLI deployment scopes synthesis to its target; interactive deployment preserves the full multi-selection. Standalone CDK synthesis still includes every target. Invalid selections fail before any stack is constructed, and missing credentials on a selected target still fail normally.

Existing generated projects retain their CDK source on upgrade; the configuration guide includes the precise update for their entry point. No construct-package update is required.

Related Issue

Closes #2569.

Documentation PR

Included in docs/configuration.md.

Type of Change

• Bug fix
• Documentation update

Testing

• 61 focused tests pass, including generated-entry-point execution, child environment isolation, shared synthesis forwarding, and interactive target selection. The core regression fails against the original entry point.
• Six local synthesis checks using the real published @aws/agentcore-cdk@0.1.0-alpha.54 pass, covering the original failure, API-key/OAuth credentials, multiple selected targets, missing selected credentials, and standalone all-target synthesis. Each generated template contains only its own target's credential ARN.
• Built unit run: 6,358 passed. Python scaffolding failures from a missing uv prerequisite were resolved by a passing 36-test rerun. Two unrelated tests still fail because their shell commands do not quote checkout paths containing spaces; reproduced on pristine upstream code.
• Local integration run: 353 passed, 3 skipped. The separate dev-server suite's setup hook timed out while creating/installing its project (120 seconds); no deployment or synthesis regression was reported.
• CLI and vended-CDK typechecks, build, formatting, whitespace, and secrets checks pass. Lint passes with existing repository warnings; changed CLI files are clean.
• Intentional asset snapshot updated; all 139 asset snapshot tests pass.

No live AWS deployment was performed.

Checklist

• I have read the CONTRIBUTING document
• I have added regression tests
• I have updated the documentation, including existing-project migration steps
• My changes generate no new warnings
• No dependent package changes are required

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