[FEATURE] Output style picker (/config) — switch output styles in-session, like Claude Code

Problem

opencode loads agent rules from AGENTS.md only, with no concept of multiple switchable "output styles". Users who want ADHD-friendly / terse / briefing variants of the agent voice (see attention-span) have to manually edit AGENTS.md to swap the style block, and cannot switch per-session.

Claude Code solves this with output-style markdown files (~/.claude/output-styles/*.md) plus a /config picker and settings.json: { "outputStyle": "..." } default. The styles are plain markdown bodies (one frontmatter name/description line each) and the engine injects the active one as a system rule.

Proposal

Add a first-class output style concept:

• ~/.config/opencode/output-styles/*.md (global) and .opencode/output-styles/ (project) — one markdown file per style, optionally with YAML frontmatter (name, description).
• A /config (or /style) slash command in the TUI that pops a list of installed styles and activates one for the current session.
• A settings.json / opencode.json key like "outputStyle": "Attention-kind" to set the default, so it persists across sessions without manual AGENTS.md edits.
• The active style body is injected into the system prompt the same way AGENTS.md content is today; non-active styles are ignored.

Why

• Per-session switching. Today, switching voice = edit AGENTS.md, restart. A picker makes it a keystroke.
• Composable with AGENTS.md. Project rules and personal voice preferences stop fighting for the same file. AGENTS.md carries project/work conventions; output styles carry delivery preferences.
• Parity with Claude Code. Users coming from Claude Code (and tools like attention-span that target it) get the same workflow on opencode with zero porting effort. The attention-span README already documents per-agent installs; a native picker would make that unnecessary.
• Cheap to implement. Markdown files + a slash command + one settings key + one system-prompt injection point. No model behavior change.

Detail

• Default-on style: none (current behavior). Backwards compatible.
• File format: plain markdown body, optional frontmatter. If no frontmatter, derive name from filename.
• Scope: global styles apply everywhere; project styles override for that project; /config selection overrides both for the session.
• Token cost: ~650 tokens per style, cached after first request — same as AGENTS.md content today.

Related

• attention-span (the styles that motivated this): https://github.com/alexgreensh/attention-span
• Claude Code output styles doc: https://code.claude.com/docs/en/output-styles