fix(#2823): auto-migrate legacy plugin name and warn users at startup

- logLegacyPluginStartupWarning now emits console.warn (visible to user,
  not just log file) when oh-my-opencode is detected in opencode.json
- Auto-migrates opencode.json plugin entry from oh-my-opencode to
  oh-my-openagent (with backup)
- plugin-config.ts: add console.warn when loading legacy config filename
- test: 10 tests covering migration, console output, edge cases

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

@@ -27,6 +27,7 @@ describe("checkForLegacyPluginEntry", () => {
     expect(result.hasLegacyEntry).toBe(true)
     expect(result.hasCanonicalEntry).toBe(false)
     expect(result.legacyEntries).toEqual(["oh-my-opencode"])
+    expect(result.configPath).toBe(join(testConfigDir, "opencode.json"))
   })
 
   it("detects a version-pinned legacy plugin entry", () => {
@@ -77,5 +78,6 @@ describe("checkForLegacyPluginEntry", () => {
     expect(result.hasLegacyEntry).toBe(false)
     expect(result.hasCanonicalEntry).toBe(false)
     expect(result.legacyEntries).toEqual([])
+    expect(result.configPath).toBeNull()
   })
 })