Release v0.1.12: unified agent chat

[marcelormendes]

Summary

• replace terminal-scraping chat with the unified engine conversation model
• stream working state, elapsed time, plans, commentary, tools, file changes, approvals, and prominent final answers
• add image attachment upload and paste chips, stable identity, follow-output behavior, and capability-based honest degradation
• render useful collapsed tool summaries and expandable command/output details, including Bash cards
• add the complete implementation plan and design specification
• bump Desktop and AUR metadata to v0.1.12

Validation

• npm run verify
• 62 test files and 510 tests passed
• TypeScript typecheck and Biome lint passed
• static-site build passed
• AUR .SRCINFO matches PKGBUILD
• live Electron/Playwright vision testing against Pi on w1:p1, with screenshots saved under /tmp/chat-test-*.png

Engine dependency

The corresponding provider-neutral engine API and adapters are in herdrdev/herdr#2672. Desktop degrades explicitly when an installed engine does not advertise agent_conversations.