feat(tui): add BTW OpenCode community plugin

[pabloXDXDXD]

Pre-flight Checklist

• I have searched existing issues and this is not a duplicate
• I understand that PRs will be rejected if the linked issue does not have status:approved

Affected Area

TUI (terminal UI)

Problem Statement

Gentle AI supports community OpenCode plugins (sub-agent-statusline, sdd-engram-plugin), but there's no plugin for parallel/background questions. When working in OpenCode, users often need to ask a follow-up question without derailing their main session — currently there's no way to do that non-blocking.

The btw-opencode plugin fills this gap: it intercepts /btw <question>, spawns a child session, fires the question in the background, and lets the user keep typing. It includes a TUI companion with /btw-list and a sidebar slot.

Proposed Solution

Add btw-opencode as a community plugin option in gentle-ai's OpenCode Plugins screen.

Changes required (3 files, ~30 lines total):

1. internal/model/types.go — add new constant:

   OpenCodePluginBtwOpenCode OpenCodeCommunityPluginID = "btw-opencode"

2. internal/components/opencodeplugin/plugin.go — add definition:

   {
       ID:          model.OpenCodePluginBtwOpenCode,
       Name:        "BTW Plugin",
       PackageName: "pabloXDXDXD/btw-opencode",
       RepoURL:     "https://github.com/pabloXDXDXD/btw-opencode",
       Owner:       "pabloXDXDXD",
       Repo:        "btw-opencode",
       Description: "Parallel /btw questions without interrupting your main session",
   }

3. internal/tui/model.go — add to plugin list and repo URLs:

   // in opencodepluginDefinitions()
   model.OpenCodePluginBtwOpenCode,
   
   // in opencodepluginRepoURLs()
   "https://github.com/pabloXDXDXD/btw-opencode"

The plugin installs via GitHub shorthand (npm install pabloXDXDXD/btw-opencode) — no npm registry auth needed. No dependencies on other plugins.

Alternatives Considered

• Manual config: users can add it to tui.json and opencode.json by hand, but gentle-ai's TUI makes discovery and registration much smoother
• Forking/inlining the plugin into gentle-ai: unnecessary, the plugin is self-contained and updates independently

Additional Context

The plugin was tested on Windows with OpenCode. Install completes in ~31s via GitHub shorthand. The gentle-ai TUI correctly lists it, registers it, and OpenCode activates both the server hook (/btw) and TUI (/btw-list) plugins. A pre-compiled binary with these changes exists for testing.