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.telemetry.test.ts

@@ -106,7 +106,7 @@ function installModuleMocks(): void {
   }))
 }
 
-describe("OhMyOpenCodePlugin telemetry isolation", () => {
+describe("oh-my-openagent telemetry isolation", () => {
   beforeEach(() => {
     mock.restore()
     installModuleMocks()

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()

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()

src/shared/log-legacy-plugin-startup-warning.test.ts

@@ -63,7 +63,7 @@ describe("logLegacyPluginStartupWarning", () => {
       //#then
       expect(mockLog).toHaveBeenCalledTimes(1)
       expect(mockLog).toHaveBeenCalledWith(
-        "[OhMyOpenCodePlugin] Legacy plugin entry detected in OpenCode config",
+        "[legacy-migration] Legacy plugin entry detected in OpenCode config",
         {
           legacyEntries: ["oh-my-opencode", "oh-my-opencode@3.13.1"],
           suggestedEntries: ["oh-my-openagent", "oh-my-openagent@3.13.1"],

src/shared/log-legacy-plugin-startup-warning.ts

@@ -22,7 +22,7 @@ export function logLegacyPluginStartupWarning(deps: LogLegacyPluginStartupWarnin
 
   const suggestedEntries = result.legacyEntries.map(toCanonicalEntry)
 
-  logFn("[OhMyOpenCodePlugin] Legacy plugin entry detected in OpenCode config", {
+  logFn("[legacy-migration] Legacy plugin entry detected in OpenCode config", {
     legacyEntries: result.legacyEntries,
     suggestedEntries,
     hasCanonicalEntry: result.hasCanonicalEntry,