feat(acp): Add Droid ACP provider

[matdev83]

feat(acp): Add Droid ACP provider

Related: https://x.com/FactoryAI/status/2016668247395877007

Current architecture / implementation constraints

Implement this as a thin external ACP connector on top of the current shared ACP runtime:

• Reuse connector-support/acp as the single ACP protocol/session/tool/stream implementation (refactor: deduplicate ACP runtime #242).
• Package Droid integration under connectors/ through the executable backend-plugin/process-host architecture (feat: add executable backend connector plugins #208); do not add a second root ACP implementation.
• Declare execution_class: agent_runtime; direct routing remains supported while feat(routing): agent-runtime execution composition safety #337 protects unsafe weighted/parallel/thinker/failover composition by default.
• Limit Droid-specific code to launch/configuration/discovery/lifecycle behavior that cannot be expressed by the shared ACP support layer.
• Preserve current cancellation, error propagation, process supervision, secret handling, and optional-dependency isolation contracts.
• Prefer connector-local contract/parity/process-tree tests and generic ACP TCK coverage over provider-specific core changes.

No dedicated Kiro SDD or implementation PR exists yet.