direct_code_deploy ignores the project .dockerignore (only the bundled template is honored)

## Summary

For `direct_code_deploy`, the source-packaging step ignores the project's own `.dockerignore` (and `.gitignore`). The ignore patterns are read **only** from the toolkit's bundled `utils/runtime/templates/dockerignore.template`, so there is no supported way to exclude project-specific directories from the uploaded `code.zip`. Any large directory not enumerated in the bundled template (e.g. `.claude/`, caches, data dirs, extra virtualenvs with non-standard names) gets packaged and uploaded.

## Where

Two code paths compute the ignore list for direct code deploy, and both hardcode the bundled template:

- `utils/runtime/package.py` → `CodeZipPackager._get_ignore_patterns()` (local zip build)
- `services/codebuild.py` → `CodeBuildService._parse_dockerignore()` — docstring even states: *"Always uses the dockerignore.template to ensure consistent file filtering during zip creation, regardless of source_path configuration."*

```python
def _get_ignore_patterns(self) -> List[str]:
# reads files("bedrock_agentcore_starter_toolkit")
# .joinpath("utils/runtime/templates/dockerignore.template")
# ...no project-level .dockerignore is consulted or merged
```

The `.dockerignore` that `operations/runtime/configure.py` / `utils/runtime/container.py` generate is only consumed by **container** deployments (Docker build context) — it is not read by the direct-code-deploy zip packagers.

There is no CLI flag (`agentcore deploy --help` has no ignore/exclude option), no `.bedrock_agentcore.yaml` key, and no environment variable to add patterns.

## Impact

A repo that keeps a large directory under `source_path` which isn't in the bundled template gets it silently zipped and uploaded to S3. In our case a 1.6 GB directory (git worktrees under `.claude/`) would have been packaged; the effective source is ~9 MB. The only workaround today is editing the installed `dockerignore.template` inside `site-packages`, which is lost on every `pip install` / fresh venv.

## Repro

1. `agentcore configure` a Python agent with `deployment_type: direct_code_deploy`.
2. Add a large directory not in the bundled template (e.g. `.claude/`, `data/`, `notebooks/`) under `source_path`.
3. Create a project-level `.dockerignore` containing that directory.
4. `agentcore deploy` → the directory is still included in `code.zip` (the project `.dockerignore` is ignored).

## Proposed fix

Have `_get_ignore_patterns()` (and `CodeBuildService._parse_dockerignore()`) **merge** the project's `.dockerignore` (resolved from `source_path` / build context) on top of the bundled template defaults. This matches Docker's own behavior and the least-surprise expectation that a `.dockerignore` in the project root is honored.

Alternatively (or additionally): support an explicit ignore list in `.bedrock_agentcore.yaml` and/or an `agentcore deploy --ignore ` flag.

## Environment

- `bedrock-agentcore-starter-toolkit` 0.3.3 (behavior confirmed identical on the current `main`, 0.3.10 — `_get_ignore_patterns` is unchanged and still reads only the bundled template)
- Deployment type: `direct_code_deploy`
- Python 3.13, macOS

Start with utils/runtime/package.py at CodeZipPackager._get_ignore_patterns() and services/codebuild.py at CodeBuildService._parse_dockerignore(), then reproduce the direct_code_deploy packaging flow with a project-level .dockerignore. Done means project patterns are honored in both code.zip paths while the bundled dockerignore.template defaults remain effective.

Written by the indexing model from the issue text.