[FEATURE]: Desktop settings: surface and edit global rules (~/.config/opencode/AGENTS.md) in the UI

Problem

OpenCode supports global personal rules via ~/.config/opencode/AGENTS.md, applied across all sessions (documented in the Rules docs, with ~/.claude/CLAUDE.md as fallback). But this mechanism is invisible in the app: nothing in the Desktop UI shows that global rules exist, where the file lives, or how to edit it. The only way to discover it is reading the docs.

Compare: Codex exposes its global instructions directly in the settings UI, so users can find and edit them without knowing file conventions.

This is especially relevant for Desktop users who never touch config files — for them, global rules effectively don't exist.

Proposal

Surface global rules in the Desktop settings UI:

1. A settings section (e.g. "Rules" or "Global instructions") that shows whether ~/.config/opencode/AGENTS.md exists and offers to create it
2. Direct editing built into the settings surface, like Codex does for its global instructions — an in-app editor pane for ~/.config/opencode/AGENTS.md, so no external tooling is needed
3. If a full in-app editor is out of scope initially, at minimum "open in editor / reveal in file manager" buttons for the file
4. A short explanation of how global and project AGENTS.md files combine, linking to the docs

Nice-to-have later: the same treatment for project-level AGENTS.md of the open workspace. Keeping this request scoped to the global file, since that is the part with no project context to anchor it.

Related reports

• #40348 — global AGENTS.md rules reportedly not reliably followed across sessions
• #35738 — /init does not honor global AGENTS.md
• #33908 — docs clarification that local and global rules are combined, not exclusive

None of these cover exposing global rules in the UI itself.

Environment

• App: OpenCode Desktop 1.18.21
• OS: Microsoft Windows 11 Education, 25H2, build 26200.8875