feat(opencode): add opencode integration — /mcpctl switcher + footer indicator

Implements mcpctl as an opencode addon, mirroring the prime-agent UX
(active-project indicator + a /mcpctl project switcher). opencode is a
native MCP client, so instead of a bridge this mounts the active project's
proxy MCP gateway through opencode's own live MCP API.

- src/opencode-ext/mcpctl-opencode.ts — server plugin (headless runs): mounts
  the active project under a stable `mcpctl` name via `client.mcp.add`,
  re-asserting on first contact and before each turn; never throws.
- src/opencode-ext/mcpctl-opencode-tui.tsx — TUI plugin: `/mcpctl`,
  `/mcpctl-status`, `/mcpctl-skills`, and a `mcpctl:<project>` indicator in
  the prompt footer (next to the model name, above the token counter); the
  switch delegates to the CLI then re-points the mount live.
- `mcpctl config opencode` — provision/reuse the project mcptoken, install +
  register the plugins (tui.json), write the 0600 state file, sync skills.
- `skills sync --agent opencode` — new shared-tree target
  (~/.config/opencode/skill).
- embedded-source generator + embed test, settings/config/order tests,
  completions, README + docs/opencode-extension.md.

Typechecked against the real @opencode-ai/plugin types (1.18.15).

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 --gateway-url --token --opencode-dir --skip-skills --skip-plugin --skip-marker --dry-run -h --help" -- "$cur"))
+            ;;
           setup)
             COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
             ;;