refactor: remove cosmetic OhMyOpenCodePlugin references

Post-V1-migration cleanup of the removed symbol's ghost references:

- src/index.ts: log prefix '[OhMyOpenCodePlugin]' -> '[oh-my-openagent]'
- src/index.test.ts: describe label 'OhMyOpenCodePlugin' -> 'oh-my-openagent plugin module'
- src/index.telemetry.test.ts: describe label 'OhMyOpenCodePlugin telemetry isolation' -> 'oh-my-openagent telemetry isolation'
- src/shared/log-legacy-plugin-startup-warning.ts: log prefix '[OhMyOpenCodePlugin]' -> '[legacy-migration]' (plus matching test assertion)

After these renames 'grep -rn OhMyOpenCodePlugin src/' returns zero matches. Pure cosmetic rename, no behavior change.

src/index.ts

@@ -20,7 +20,7 @@ import { createPluginPostHog, getPostHogDistinctId } from "./shared/posthog"
 
 const serverPlugin: Plugin = async (input, _options): Promise<Hooks> => {
   initConfigContext("opencode", null)
-  log("[OhMyOpenCodePlugin] ENTRY - plugin loading", {
+  log("[oh-my-openagent] ENTRY - plugin loading", {
     directory: input.directory,
   })
   logLegacyPluginStartupWarning()