fix: mark Kiro as safe for multi-install

[NgoQuocViet2001]

Description

Kiro writes its generated commands under the isolated .kiro/prompts tree, but its integration did not declare the existing multi-install safety capability. As a result, compatible co-installs were reported as permanently unsafe and required --force.

This change:

• marks KiroCliIntegration as multi_install_safe;
• adds a dedicated regression test so removing the declaration fails independently of the registry matrix.

Closes #3471.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Additional validation:

• uv sync --extra test
• Kiro integration and agent-metadata tests: 51 passed
• Full tests/integrations/test_registry.py: 365 passed
• Multi-install safety contract subset: 290 passed
• Manual Claude + Kiro co-install without --force: integration status OK, Multi-install safe: yes, and no unsafe-multi-install finding
• Ruff on the production file and git diff --check: passed

The full repository test suite was not run, so that checkbox is intentionally left unchecked.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Codex (GPT-5, autonomous) analyzed the issue and repository contracts, authored the code and regression test, and ran the validations above on behalf of @NgoQuocViet2001. The operator requested this contribution but did not manually author or line-by-line review the patch.