feat(opencode): native opencode addon — /mcpctl switcher, live project switching, footer indicator

Implement mcpctl as an opencode addon mirroring the pi/prime-agent
integrations. Mounts the active project's MCP gateway through opencode's own
live MCP API, so switching projects needs no restart and opencode.json is
never touched (the bearer token stays in a 0600 state file).

- server plugin (plugin/mcpctl.ts): headless mount under a stable 'mcpctl'
  name, re-asserted on first contact + before each turn; skips re-registering
  an unchanged mount so a gated project's begin_session state survives.
- TUI plugin (mcpctl/mcpctl-tui.tsx): /mcpctl filterable picker (live switch),
  /mcpctl-status, /mcpctl-skills, and a mcpctl:<project> footer indicator
  right of the model name, directly above the token counter.
- mcpctl config opencode: mint/reuse project mcptoken, install + register
  plugins, write 0600 state, write marker, sync skills.
- skills sync --agent opencode: new shared-tree target (XDG-aware).
- shared credential plumbing lifted from config prime-agent and parameterised
  by agent so both hosts share mint/reuse/retire logic.
- embedded-source generator + freshness test; typechecked against the real
  @opencode-ai/plugin types (1.18.15); unit tests for settings, order, embed.

typecheck (incl. opencode-ext) and the full cli suite (619 tests) pass.

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"))
             ;;