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.test.ts

@@ -110,7 +110,7 @@ async function importFreshIndexModule(): Promise<typeof import("./index")> {
   return import(`./index?test=${Date.now()}-${Math.random()}`)
 }
 
-describe("OhMyOpenCodePlugin", () => {
+describe("oh-my-openagent plugin module", () => {
   beforeEach(async () => {
     mock.restore()
     installIndexModuleMocks()