Add AI stance and capabilities feature page

[atlas-maxjb]

Summary

• Add /features/ai as the replacement for /features/assistant.
• Lead with Sure's opt-in, user-controlled AI stance.
• Document the AI functionality currently implemented in Sure: Assistant, transaction categorization, merchant detection, generated insights, and PDF/document processing.
• Explain built-in, external, OpenAI, Anthropic, and custom OpenAI-compatible/local-provider paths.
• Redirect legacy AI/Assistant routes to the new page.
• Link CTAs to the open-source repository.

Verification

• git diff --check passed.
• HTML structure checks passed.
• Rails tests could not run in this environment because Ruby is unavailable.
• No production deployment or merge performed.

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