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

Adds `mcpctl config opencode`, two opencode plugins and an `opencode` skills
sync target, so an mcpctl project can be switched from inside opencode's TUI
and the active one is visible at a glance.

Unlike `config claude` / `config prime-agent`, this writes NO MCP entry into
the host's config. opencode exposes an HTTP API for its own MCP registry
(`POST /mcp`), so the project is mounted through the running app:

  - the token stays in ~/.mcpctl/opencode-state.json (0600) instead of a
    mode-0644 opencode.json users paste into bug reports;
  - switching projects takes effect on the next turn, with no restart.

Inside opencode:
  /mcpctl         filterable project picker; switches live
  /mcpctl-status  active project, mount state, gateway URL
  /mcpctl-skills  re-sync this project's skills
  plus a `mcpctl:<project>` indicator in the prompt footer, next to the model
  name and one line above the token counter.

Design notes:
  - the MCP server is registered under a constant name, so tools keep a stable
    `mcpctl_*` prefix and opencode's per-request tool resolution shows the new
    project's tools by itself — no "your old tool names are dead" message to
    the model, unlike the pi extension;
  - an unchanged mount is never re-registered: mcp.add rebuilds the connection
    and mcplocal binds a gated project's unlocked state to that connection's
    mcp-session-id, so re-adding would re-lock a project begin_session had just
    opened;
  - the server plugin does not mount during setup — setup runs before the
    server accepts connections and mcp.add calls back into it, which hangs
    opencode on a blank screen before the TUI draws;
  - the switcher shells out to this CLI (--skip-plugin --skip-marker) so token
    minting, state and skills stay in one place;
  - no usable credential aborts non-zero with the state file untouched, so a
    failed switch leaves the previous project working rather than swapping it
    for a mount that 401s.

`skills sync --agent opencode` installs into ~/.config/opencode/skill (XDG
aware) with the same shared-tree semantics as pi and prime-agent. The
credential plumbing shared with `config prime-agent` is lifted to one place and
parameterised by agent rather than copied.

The plugin sources are embedded in the CLI (generated, freshness-tested) so an
installed binary with no source tree can provision them, and are typechecked
against the real @opencode-ai/plugin types.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVwuCjuMoA13gmzYEfcrNP

README.md

@@ -195,6 +195,48 @@ Inside pi:
 
 See [docs/pi-extension.md](docs/pi-extension.md) for full details.
 
+### Connect opencode
+
+[opencode](https://opencode.ai) supports MCP natively *and* exposes an API for
+its own MCP registry, so mcpctl mounts the project through the running app —
+which means you can **switch projects from the TUI without restarting**.
+
+```bash
+mcpctl config opencode --project monitoring
+```
+
+This mints (or reuses) the project's gateway token into
+`~/.mcpctl/opencode-state.json` (0600 — the token never lands in
+`opencode.json`), installs a server plugin into
+`~/.config/opencode/plugin/mcpctl.ts` and a TUI plugin into
+`~/.config/opencode/mcpctl/`, registers the latter in
+`~/.config/opencode/tui.json`, and syncs skills into
+`~/.config/opencode/skill/`.
+
+Inside opencode:
+
+- `/mcpctl` — **switch project** from a filterable picker; takes effect on the
+  next turn, no restart
+- `/mcpctl-status` — active project, mount state, gateway URL
+- `/mcpctl-skills` — re-sync this project's skills
+- the active project shows as `mcpctl:<project>` in the prompt footer, next to
+  the model name
+
+Skip individual steps as needed:
+
+```bash
+mcpctl config opencode --project monitoring --token mcpctl_pat_xxx  # provide token, don't mint
+mcpctl config opencode --project monitoring --skip-skills           # don't sync skills
+mcpctl config opencode --project monitoring --skip-plugin           # state only, leave plugins alone
+mcpctl config opencode --project monitoring --skip-marker           # don't touch .mcpctl-project here
+mcpctl config opencode --project monitoring --dry-run               # print the plan, write nothing
+```
+
+Re-sync skills on their own with
+`mcpctl skills sync --agent opencode --project monitoring`.
+
+See [docs/opencode-extension.md](docs/opencode-extension.md) for full details.
+
 ## Declarative Configuration
 
 Everything can be defined in YAML and applied with `mcpctl apply`: