feat(harness-app): support runtime config overrides - #1052

Summary

• Add HarnessApp runtime config override support for model params, tools, skills, MCP, knowledge base, long-term memory, and max LLM calls.
• Add AgentKit resource resolvers for knowledge base, long-term memory, and MCP router ids.
• Extend HarnessApp HTTP/runtime behavior for session creation, /get_agent_config, harness_merge, and request-level plugin config.
• Update HarnessApp image dependencies to include optional backend extras needed by request-level resources.

Tests

• uv run pre-commit run --all-files
• uv run pytest -q tests/cli/test_cli_harness_open_source_defaults.py tests/cloud/test_harness_app_contract.py tests/cloud/test_harness_app_http.py tests/cloud/test_harness_enhance_env.py tests/tools/test_builtin_registry_contract.py