Add optional AI assistant and harden releases

[goldwav]

What changed

• adds an optional bring-your-own-key AI assistant for OpenAI, Anthropic, and OpenRouter
• adds quick actions for prioritizing tasks, rewriting, code improvement, review, and custom instructions
• protects credentials for the current Windows user and keeps note text behind an explicit Send action
• previews every proposed replacement with stale-note and stale-selection safeguards
• hardens provider networking, error handling, response limits, accessibility state, and cancellation
• strengthens portable-release provenance, content hashing, and backward-compatible verification

Why

The Notepad should offer modern assistance without making AI a dependency or quietly changing the local-first editing model. Users remain in control of the provider, model, key, data sent, and whether any proposed edit is applied.

Verification

• native compatibility build: 49 passed, 0 failed
• full WPF application compiled
• clean-source 0.2.0-ai-preview.4 portable package built and independently verified
• historical schema-v1 release artifacts still verify
• staged source scanned for secrets, local paths, and conversation/tool traces
• security diff audit found no remaining code or credential-handling blocker

Release boundary

This remains an unsigned engineering preview. Live provider requests, packaged Narrator/high-contrast/DPI testing, runtime network capture, code signing, and clean-machine certification remain explicit manual gates.

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