feat: add starter-brain seeding with idempotent 10-rule defaults

[Gradata]

Implements GRA-220 starter-brain feature: installer seeds ~/.gradata/brain/rules/starter.json with 10 safe default rules when the feature flag is enabled.

What

• Feature flag GRADATA_STARTER_BRAIN env var or [starter_brain] enabled = true in config.toml (defaults to false)
• seedStarterBrain() writes 10 safe starter rules in JSON format
• Idempotent: re-running install skips if all 10 rule IDs are already present
• Existing install behavior unchanged when flag is disabled

Changes

• setup/install.js: added isStarterBrainEnabled(), STARTER_RULES constant, seedStarterBrain(), and call in main()

Test evidence

• node --check setup/install.js passes (exit 0)

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