feat(harness): add single-wheel sidecar extension

[zyn080302]

Summary

• add agentkit.extensions.harness_sidecar to the existing AgentKit SDK wheel
• keep the private Sidecar Runtime out of public package dependencies; managed AgentKit runtimes supply it
• expose Sidecar/VPC deployment through agentkit harness sidecar deploy
• keep legacy agentkit deploy --harness behavior unchanged through opt-in core extension hooks

Compatibility

• existing core imports and CLI options are unchanged
• new deploy hooks are optional, keyword-only, and default to None
• the harness-sidecar extra is a stable selector and does not create a second wheel

Verification

• Sidecar and compatibility tests: 66 passed
• remaining repository tests outside baseline apps/frameworks dependency gaps: 925 passed
• Ruff, formatting, secret scan, and git diff --check: passed
• built one agentkit-sdk-python wheel; verified Sidecar modules and extra metadata are present and no private Runtime dependency is declared