Feature: Add a new slash command

[quiet-node]

Label: enhancement, good first issue
Milestone:
Projects:

Problem Statement

Thuki already has useful slash commands like /tldr, /rewrite, and /bullets. A /explain command would let users highlight any text (code, jargon, a paragraph) and instantly get a plain-English explanation, without having to type out a prompt manually.

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Proposed Solution

Add a /explain command to the slash command registry in src/config/commands.ts.

The command should:

• Accept highlighted text or typed text as input
• Ask the model to explain it in plain, simple language
• Output only the explanation (no preamble like "Sure! Here's an explanation...")

All the plumbing is already in place. Adding a command here is all that's needed: the autocomplete UI, the tab-completion, and the prompt builder all pick it up automatically.

File to edit: src/config/commands.ts

Follow the exact same shape as the existing entries (e.g. /tldr or /bullets). Each command needs:

• trigger — the slash string, e.g. "/explain"
• label — same as trigger
• description — one-line hint shown in the autocomplete dropdown
• docs — human-facing documentation fields (summary, usage, examples, behavior)
• promptHelp — short model-facing summary
• promptTemplate — the actual prompt sent to the model, using $INPUT as the placeholder for the user's text

A suggested prompt template to start from:

Explain the following in plain, simple language. Assume the reader is smart but not familiar with the topic. Be concise. Output only the explanation, no introduction or sign-off.

Text: $INPUT

Feel free to improve the wording.

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Definition of Done

• /explain appears in the COMMANDS array in src/config/commands.ts
• Typing /explain in the ask bar shows it in the autocomplete suggestions
• Submitting /explain with highlighted or typed text sends the prompt to the model and streams a response
• bun run test:all:coverage passes (100% coverage maintained)
• bun run validate-build passes with zero warnings and zero errors