fix: add opt-out for embedding model download

[pszymkowiak]

Summary

• Add 3 ways to disable embeddings entirely (skip model download at startup):
  • Config: [embeddings] enabled = false
  • CLI flag: icm --no-embeddings <command>
  • Env var: ICM_NO_EMBEDDINGS=1
• When disabled, ICM falls back to FTS5 keyword search (still functional, no semantic matching)
• Update README and default config with documentation

Closes #8

Test plan

• cargo test — 147/147 pass
• icm --no-embeddings stats works without model download
• ICM_NO_EMBEDDINGS=1 icm recall "test" returns FTS results
• Default behavior unchanged (embeddings enabled)

[pszymkowiak]

🧞 wshm · Automated triage by AI

📊 Automated PR Analysis

✨ Type	feature
🟢 Risk	low

Summary

Adds three mechanisms (config flag, CLI flag, environment variable) to opt out of embedding model download at startup, falling back to FTS5 keyword search when disabled. Updates README and default config with documentation for the new options.

Review Checklist

• Tests present
• Breaking change
• Docs updated

Linked issues: #8

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🤖 Analyzed automatically by wshm · This is an automated analysis, not a human review.