feat: error messages are not actionable, how to debug CDK synth failures and container build failures?

### Description

When using agentcore-cli, I ran into two issues where the error output was too vague to act on.

**Issue 1: CDK synth fails silently**

After initialising a new agentcore project and running `agentcore memory import`, I got:

```
CDK synth failed: node dist/bin/cdk.js: Subprocess exited with error 1
```

No further detail. The root cause turned out to be that `cdk bootstrap` had not been run yet. A
clearer error like "CDK environment is not bootstrapped, run `cdk bootstrap` first" would have
saved a lot of time.

**Issue 2: Container build fails silently**

Running `agentcore dev` produced:

```
container build failed (exit code 1) agentcore
```

Again, no log output. Workaround was to run the agent directly (`uv run main.py`) and skip the
CLI wrapper entirely.

**Questions:**
1. Where are the full build logs for `agentcore dev` stored so I can diagnose container build
failures?
2. Is there a verbose or debug flag for agentcore-cli?
3. Is `cdk bootstrap` an undocumented prerequisite for all agentcore-cli commands, or only
deployment-related ones?

### Acceptance Criteria

1. Explains where the full logs for `agentcore dev` container build failures are stored, or how to retrieve them.
2. Confirms whether a verbose or debug flag exists for agentcore-cli, and if so, how to use it.
3. Clearly states whether `cdk bootstrap` is a prerequisite for all agentcore-cli commands or only a subset, and ideally links to official documentation that mentions this requirement.

Optional: explains the recommended workflow order when setting up a new agentcore project for
the first time (e.g. bootstrap before any other command).

### Additional Context

_No response_