fix: keep devcontainer setup running after Kiro failure

[wondr-wclabs]

Summary

Fixes #2605.

The devcontainer post-create.sh script used the fatal run_command wrapper for every Kiro installer step. When the upstream Kiro installer URL returns 404, that exits the whole post-create script and prevents later tools from installing, including Kimi CLI, CodeBuddy, uv, and DocFx.

This keeps the pinned Kiro installer checksum verification in place when the installer is available, but makes the Kiro block non-blocking:

• adds run_optional_command for installer steps that should warn and continue;
• applies it only to the Kiro install/check flow;
• preserves fatal behavior for the rest of the setup script;
• adds a regression check that the Kiro block does not contain a direct exit 1 and advertises continued setup.

The reason for limiting the optional behavior to Kiro is that #2605 is caused by one unavailable optional upstream installer. Making every devcontainer install non-fatal would hide real setup failures for core tooling.

Validation

• bash -n .devcontainer/post-create.sh
• uv run python -m pytest tests/test_agent_config_consistency.py -q — 30 passed
• uv run python -m pytest tests/integrations/test_integration_kiro_cli.py tests/test_agent_config_consistency.py -q — 55 passed
• git diff --check
• uv run python -m pytest -q — 3576 passed, 45 skipped

AI assistance disclosure

This PR was developed with AI assistance from Codex/ChatGPT for codebase inspection, implementation, and validation. I reviewed the diff and test output before submitting.