feat(ui,cli): make task title optional with AI auto-generation

[bborn]

Summary

• Add GenerateTitle function to autocomplete service for AI-powered title generation from task descriptions
• Update form placeholders to indicate title is optional when details are provided
• Show "✨ AI will generate title" indicator in the UI when title is empty but body has content
• CLI create command now accepts 0 or 1 positional args - title is optional with --body flag
• Both UI and CLI use the same fallback when AI is unavailable: first line of body truncated at 50 chars

How it works

1. UI: When creating a new task, if the user leaves the title empty but provides details in the body field, an indicator shows that AI will generate the title on submission
2. CLI: task create --body "The login button is broken" will auto-generate a title like "Fix login button"
3. Fallback: If no API key is configured or the LLM call fails, uses the first line of the body (truncated) as the title

Test plan

• Build passes: go build ./...
• All existing tests pass: go test ./...
• New tests added for GenerateTitle and buildTitleGenerationPrompt functions
• Manual test: Create task via UI with only details, verify AI generates title
• Manual test: Create task via CLI with --body flag only, verify AI generates title
• Manual test: Verify fallback works when API key is not configured

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