feat(plugins): backend-engine-definition plugin (Phase 5 #47)

[charlie83Gs]

Summary

• First Phase-5 provider plugin: plugins/backend-engine-definition/
• Contributes default NodeDefinitionProvider matching GraphTypeComposition.definition on the default graph type
• Registered in kt_config.plugin.load_default_plugins alongside existing built-in plugins

What changes

• New DefaultNodeDefinitionProvider wraps the LLM definition-synthesis flow today's DefinitionPipeline.generate_definition runs inline — registry-driven dispatch resolves to an identical prompt/LLM call, zero behaviour change for existing graphs.
• Provider is stateless: caller pre-fetches dimensions and passes them via NodeDefinitionContext.options['dimensions']. Keeps a graph-engine handle out of the ABC while preserving the legacy skip-path contract (no dims / empty LLM output / gateway exception → returns None, caller keeps previous definition).
• No worker wiring in this PR — the DefinitionPipeline call sites continue to run the legacy path. A follow-up will thread plugin_registry.get_definition_provider through every consumer (mirrors the P4 fact-decomposition plugin-then-wire sequencing: feat(plugins): backend-engine-fact-decomposition plugin (Phase 4 #44) #279 → feat(frontend): earth-tone theme, dark/light toggle, synthesis page improvements #44).

Test plan

• uv run --project plugins/backend-engine-definition pytest -x -v (10 contract tests green)
• uv run --project libs/kt-config pytest -x -v (236 green — registration doesn't regress registry)
• uv run --frozen ruff format --check . / ruff check .
• Pre-push hook ran full unit test suite across 14 libs + 7 services + 2 plugins — all green

🤖 Generated with Claude Code

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