feat(opencode): native opencode addon — bottom-row project indicator + in-TUI switcher

Adds an opencode integration mirroring the existing pi and prime-agent addons.

The addon is a TUI plugin (`src/opencode-ext/open-mcpctl.tsx`) that:
- renders the active mcpctl project into opencode's `app_bottom` slot — the
  status row that also carries the token counter / model info,
- registers a `mcpctl.switch_project` command bound to `<leader>m` that opens a
  searchable `DialogSelect` project picker,
- switches live by re-pointing opencode's `mcpctl` MCP server at the new
  project's mcplocal endpoint via the SDK client (native MCP, no JSON-RPC code),
- persists state in `~/.mcpctl/opencode-state.json` (+ `.mcpctl-project` marker).

Pure helpers (`filterProjects`, `toolChangeAnnouncement`) live in
`projects.ts` so they are unit-tested.

`mcpctl config opencode --project X` provisions it: writes the plugin into
`~/.config/opencode/plugin/`, registers the `.tsx` in opencode.json's plugin
array (auto-discovery only matches {ts,js}), best-effort installs the TUI peer
deps (`@opentui/*`), and syncs skills into `~/.config/opencode/skill`.
`mcpctl skills sync --agent opencode` is added as a fourth shared-tree target.

Closes typecheck gap with `typecheck:opencode-ext` against the real
`@opencode-ai/plugin` + `@opentui/solid` types.

Tests: embed freshness, opencode-settings, projects helpers, skills agent
root. Full `pnpm run typecheck` + cli tests pass; completions regenerated.

completions/mcpctl.bash

@@ -103,7 +103,7 @@ _mcpctl() {
     config)
       local config_sub=$(_mcpctl_get_subcmd $subcmd_pos)
       if [[ -z "$config_sub" ]]; then
-        COMPREPLY=($(compgen -W "view set path reset claude claude-generate pi prime-agent prime-agent-generate setup impersonate help" -- "$cur"))
+        COMPREPLY=($(compgen -W "view set path reset claude claude-generate pi prime-agent prime-agent-generate opencode setup impersonate help" -- "$cur"))
       else
         case "$config_sub" in
           view)
@@ -133,6 +133,9 @@ _mcpctl() {
           prime-agent-generate)
             COMPREPLY=($(compgen -W "-p --project -o --output --gateway-url --token --skip-skills --skip-extension --skip-marker --dry-run -h --help" -- "$cur"))
             ;;
+          opencode)
+            COMPREPLY=($(compgen -W "-p --project --extension-dir --skip-deps --skip-skills --opencode-dir -h --help" -- "$cur"))
+            ;;
           setup)
             COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
             ;;