Make Docker the default runtime and update docs for macOS + Linux support

[jmgilman]

Motivation

• Default the runtime to Docker to match new integration test coverage and CI expectations.
• Reflect expanded platform support (macOS and Linux) across docs and ADRs now that Linux runtimes are supported.
• Ensure runtime defaults and manager fallbacks are consistent with the new cross-platform posture.

Description

• Changed the default runtime in the configuration loader from podman to docker (internal/config/config.go).
• Updated manager fallback to use RuntimeDocker when no runtime is specified and clarified the ManagerConfig comment (internal/instance/manager.go).
• Updated CLI/runtime selection defaults to prefer Docker and adjusted the runtime binary mapping in internal/cmd/root.go.
• Revised documentation and ADRs to indicate macOS and Linux support and to make Docker the documented default, including README.md, docs/*, and docs/reference/configuration.md examples, and updated the manager unit test expectation in internal/instance/manager_test.go.

Testing

• Updated unit test TestNewManager to expect RuntimeDocker as the default; the test file was adjusted accordingly.
• No automated test suite (unit or integration) was run as part of this change.
• Integration test harness and scripts were left intact and continue to support docker, podman, and apple runtimes for CI use.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	headjack	3234e76	Commit Preview URL Branch Preview URL	Jan 03 2026, 08:13 PM