Design for Agent Threads v0.4.0

Features

• /design now suggests its five mode keywords as you type (wireframe:, states:, variations:, pick:, spec:), each with a one-line description — matching how /model already suggests fable|opus|.... Typing /design wire surfaces wireframe: as a completion. (#8)
• On a brand-new thread's dispatch entry point, only the three modes that make sense there are suggested (wireframe:, states:, variations:) — pick: and spec: are omitted since both always error with nothing yet to pick or annotate.

Notes

• Requires the companion Agent Threads host field SlashCommandHandler.argCompletions, shipped in agent-threads v0.51.0. This release is additive and backward-compatible: the field is optional and degrades silently on an older host build — no autocomplete appears, but /design otherwise behaves exactly as before. Confirmed empirically against a pre-change host build, not just asserted.
• Per-mode descriptions were consolidated into a single shared source (DESIGN_MODE_DESCRIPTIONS in designModes.ts), removing prior duplication between the tool schema and the new completions.
• Interactively verified end-to-end in headless Geode across both composer surfaces (thread and dispatch), plus a backward-compatibility check against a pre-change agent-threads build — this was hands-on QA, not just unit tests.
• Download both main.js and manifest.json to install or update the plugin.
• Verification: 146 tests passed, TypeScript typecheck clean, and the production build clean — all observed on main after merging #8.