[FEATURE]: Add /skills command and skill panel to Web/Desktop app #26297

Problem

The TUI has a dedicated /skills dialog (DialogSkill) that lists all available skills and allows searching/selecting them. The Web and Desktop apps lack this entirely — there is no way to:

1. List all available skills from the UI
2. Browse/search skills before invoking them
3. See which skills are currently loaded

The / slash autocomplete in the Web/Desktop prompt only shows builtin + custom commands, but not skills discovered from .opencode/skills/, .claude/skills/, or .agents/skills/.

Background

• TUI: Has DialogSkill component at packages/opencode/src/cli/cmd/tui/component/dialog-skill.tsx that calls sdk.client.app.skills() — works well.
• App (Web/Desktop): The slash-popover.tsx has a SlashCommand type that already supports source?: "command" | "mcp" | "skill", but skills are never populated into the slash command list. The prompt-input.tsx only pulls from sync.data.command (custom commands) and command.options (built-in).

Request

1. Add /skills command to Web/Desktop that opens a searchable skill list (similar to TUI DialogSkill)
2. Show skills in / autocomplete — skills should appear alongside built-in and custom commands when typing /
3. Skill management panel — optionally, a dedicated panel/settings page to browse installed skills, view their descriptions, and enable/disable them

Related Issues

• [FEATURE]: Add /skills command to list and quick-invoke skills #7846 — Add /skills command to list and quick-invoke skills (TUI focus)
• [FEATURE]: display loaded skills in status page and sidebar #7533 — Display loaded skills in status page and sidebar
• Bug: Custom skills not shown in / autocomplete menu #25117 — Custom skills not shown in / autocomplete menu
• fix(tui): show skills in autocomplete dropdown (#22129) #23987 — fix(tui): show skills in autocomplete dropdown (PR pending for TUI)
• Opencode dekstop application is not using skills #15813 — Desktop not using skills at all (closed)

Would the maintainers be open to a PR for this? Happy to implement if the approach is agreed upon.