Record build tool versions in exported bundle metadata

[stikves]

Adds a build_info section to metadata.json capturing the versions of coreai-core, coreai-torch, coreai-opt, coreai-models, torch, and transformers used during export. This makes bundles self-documenting for debugging version-specific issues.

Example metadata.json output:

  "build_info": {
    "coreai-core": "1.0.0b1",
    "coreai-torch": "0.4.0",
    "coreai-opt": "0.2.0",
    "coreai-models": "0.1.0",
    "torch": "2.9.0",
    "transformers": "4.57.6"
  }

Verified still accepted by the llm-runner:

% ./llm-runner --model exports/qwen3_0_6b_4bit_dynamic

⏳ Preparing AI asset from source... done in 1.160s

Generating...
<think>
Okay, the user is asking, "Hello, how are you?" I need to respond in a friendly and helpful way. Let me start by acknowledging their greeting. Since I'm an AI assistant, I should be polite and make sure to

⏱️  Performance Summary:
==================================================
Model Load: 1160.5ms
Prompt:     29.8ms, 14 tokens, 469.1 tokens/sec
Generation: 158.7ms, 50 tokens, 315.1 tokens/sec
Total:      1.539s
==================================================