test(opencode): cover the opencode2 host-env branch and stop inheriting ORCA_OPENCODE_AGENT (#22547)

* test(opencode): pin per-major OpenCode overlay selection on WSL and the relay

OpenCode 2 dies with "Duplicate plugin ID" when two plugin files share an id,
so which overlay a guest or remote pane is pointed at decides whether the agent
starts. Nothing failed if that selection regressed:

- the WSL spawn path never asserted which major it asks the guest relay for,
  and the shared pty-ipc mock had no openCode2HookService at all, so no test
  could reach the opencode2 branch of buildPtyHostEnv;
- requestGuestOpenCodeOverlayDir had no coverage for the v2 guest dir;
- PluginOverlayManager had no case for a remote config root that still holds
  the other major's stale Orca plugin.

Tests only; no behavior change. Each new case was mutation-checked against the
production line it guards.

* test(opencode): stop the plugin contract test inheriting ORCA_OPENCODE_AGENT

The generated plugin self-disables when ORCA_OPENCODE_AGENT names a different
major, and the contract test saved and restored that variable without ever
setting it. Run from a shell that has it — which is any shell inside an Orca
OpenCode pane, i.e. how this repo is usually developed — the plugin returned an
empty hook set and the contract failed for the wrong reason.

Delete it in beforeEach, the way the opencode2 setup test already pins it.
Verified the file passes with the variable set to either major and unset;
before this it failed for two of the three.

src/main/opencode/hook-plugin-module-contract.test.ts

@@ -52,6 +52,10 @@ describe('OpenCode status plugin module contract', () => {
     for (const key of ENV_KEYS) {
       savedEnv[key] = process.env[key]
     }
+    // Why: the generated plugin self-disables when this names a different major,
+    // so an inherited value from the developer's own Orca pane would leave
+    // `hooks.event` undefined and fail the contract for the wrong reason.
+    delete process.env.ORCA_OPENCODE_AGENT
     delete process.env.ORCA_AGENT_HOOK_ENDPOINT
     process.env.ORCA_AGENT_HOOK_PORT = '59999'
     process.env.ORCA_AGENT_HOOK_TOKEN = 'test-token'