feat: redesign /rewrite for a natural, casual voice

[quiet-node]

Overview

Redesigns the /rewrite slash command so it rewrites text into a natural, casual, everyday voice instead of lightly polishing it. The previous prompt only smoothed flow with minimal edits; the new prompt rewrites freely to sound like a fluent native speaker in everyday conversation while preserving the writer's meaning, personality, and point of view.

What changed

• New /rewrite promptTemplate in src/config/commands.ts: defaults to a relaxed, conversational tone and only stays formal when the source clearly is, never making the text stiffer than the original.
• Preserves the writer's voice: keeps expressive spellings ("heyyyy", "soooo"), contractions, slang, and any emoji the user wrote, and never adds energy, slang, or emoji the user did not use.
• Preserves point of view: never invents a narrator and never turns an imperative or impersonal note into a "we" voice; each instruction stays a direct command.
• Honors the existing [Additional instruction] follow-up channel, so tweaks like "make it more formal" override the defaults.
• Output stays clean: no em dashes, no markdown, no decorative icons, only the rewritten text.
• Refreshes the user-facing copy: command description, docs summary and behavior, and the prompt-help summary.
• Regenerates the derived artifacts (docs/commands.md, src-tauri/prompts/generated/slash_commands.txt) from source via generate:commands.

How it works

/rewrite keeps the same unified dispatch and $INPUT substitution path; only the prompt content changed. Selected text plus a typed remainder still compose into selection\n\n[Additional instruction]: remainder, which the new prompt explicitly respects so follow-up tweaks keep working.

Testing

• bun run test:coverage: 1521 frontend tests pass; the /rewrite prompt assertions in App.test.tsx were updated to the new template.
• bun run typecheck, bun run lint:frontend, bun run format:check, bun run build:frontend: all clean.
• Generated-artifact parity is enforced by commandArtifacts.test.ts.

Note: the backend is untouched apart from the regenerated prompt string that is baked in via include_str!; no Rust logic or test depends on its exact wording.