P1.2 · Settings UI: API key, team key, target folder

What

PluginSettingTab with three fields:

• Linear API key (masked input)
• Team key (e.g. KAI) — text input
• Target folder path — folder suggester

Acceptance criteria

• Settings persist across vault reloads (via this.saveData() / this.loadData()).
• On API key blur, run a live validation call to Linear's viewer { id, name } query and show a ✓ or ✗ badge.
• Folder suggester autocompletes against the vault's folder tree.
• Zod schema validates settings shape on load; invalid settings fall back to defaults.

Security

• API key stored via this.saveData() only; never sent anywhere except to api.linear.app/graphql.
• Document this clearly in the settings UI and README.