fix: resolve 3 bugs (#3366, #3272, #3222)

- shell-env: detect Git Bash via MSYSTEM env var when SHELL is unset (#3366)
  On some Git Bash installations SHELL is not set but MSYSTEM (MINGW64/MSYS)
  is always present. Check MSYSTEM before PSModulePath to avoid emitting
  PowerShell syntax in bash shells.

- session-state: resolve legacy agent names in resolveRegisteredAgentName (#3272)
  Historical sessions stored agent names like 'Sisyphus (Ultraworker)' which
  don't match the current registered format. Fall back to getAgentConfigKey
  for legacy/parenthesized name resolution before returning the raw name.

- config-migration: skip backup when file content is unchanged (#3222)
  Compare serialized config with existing file content before creating a
  timestamped .bak file. Only create backup when the on-disk content
  actually differs from the migrated content.

src/shared/migration/config-migration.test.ts

@@ -119,3 +119,52 @@ describe("migrateConfigFile sidecar write ordering", () => {
     expect(statSync(getSidecarPath(configPath)).isDirectory()).toBe(true)
   })
 })
+
+describe("migrateConfigFile backup skipping", () => {
+  test("skips backup when file content is identical after migration", () => {
+    // given - config with legacy key that migrates to same on-disk content
+    const workdir = createWorkdir()
+    const configPath = join(workdir, "oh-my-opencode.json")
+    const migratedContent = {
+      disabled_hooks: ["comment-checker"],
+    }
+
+    // Write the already-migrated content to disk
+    writeFileSync(configPath, JSON.stringify(migratedContent, null, 2) + "\n")
+
+    // rawConfig still has the legacy hook that will be removed
+    const rawConfig: Record<string, unknown> = {
+      disabled_hooks: ["gpt-permission-continuation", "comment-checker"],
+    }
+
+    // when
+    migrateConfigFile(configPath, rawConfig)
+
+    // then - no backup file should be created since file content is unchanged
+    const files = require("fs").readdirSync(workdir) as string[]
+    const backupFiles = files.filter((f: string) => f.includes(".bak."))
+    expect(backupFiles.length).toBe(0)
+  })
+
+  test("creates backup when file content actually changes", () => {
+    // given - config with model that needs migration
+    const workdir = createWorkdir()
+    const configPath = join(workdir, "oh-my-opencode.json")
+    const rawConfig = {
+      agents: {
+        prometheus: { model: "anthropic/claude-opus-4-5" },
+      },
+    }
+
+    writeFileSync(configPath, JSON.stringify(rawConfig, null, 2) + "\n")
+
+    // when
+    const needsWrite = migrateConfigFile(configPath, rawConfig as Record<string, unknown>)
+
+    // then - backup should be created since content changed
+    expect(needsWrite).toBe(true)
+    const files = require("fs").readdirSync(workdir) as string[]
+    const backupFiles = files.filter((f: string) => f.includes(".bak."))
+    expect(backupFiles.length).toBe(1)
+  })
+})