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/shell-env.test.ts

@@ -10,6 +10,7 @@ describe("shell-env", () => {
     originalEnv = {
       SHELL: process.env.SHELL,
       PSModulePath: process.env.PSModulePath,
+      MSYSTEM: process.env.MSYSTEM,
     }
   })
 
@@ -47,6 +48,7 @@ describe("shell-env", () => {
 
     test("#given PSModulePath is set without SHELL #when detectShellType is called #then returns powershell", () => {
       delete process.env.SHELL
+      delete process.env.MSYSTEM
       process.env.PSModulePath = "C:\\Program Files\\PowerShell\\Modules"
       Object.defineProperty(process, "platform", { value: "win32" })
 
@@ -58,6 +60,7 @@ describe("shell-env", () => {
     test("#given Windows platform without PSModulePath #when detectShellType is called #then returns cmd", () => {
       delete process.env.PSModulePath
       delete process.env.SHELL
+      delete process.env.MSYSTEM
       Object.defineProperty(process, "platform", { value: "win32" })
 
       const result = detectShellType()
@@ -68,6 +71,7 @@ describe("shell-env", () => {
     test("#given non-Windows platform without SHELL env var #when detectShellType is called #then returns unix", () => {
       delete process.env.PSModulePath
       delete process.env.SHELL
+      delete process.env.MSYSTEM
       Object.defineProperty(process, "platform", { value: "linux" })
 
       const result = detectShellType()
@@ -94,6 +98,28 @@ describe("shell-env", () => {
 
       expect(result).toBe("unix")
     })
+
+    test("#given MSYSTEM set on Windows without SHELL #when detectShellType is called #then returns unix", () => {
+      delete process.env.SHELL
+      process.env.MSYSTEM = "MINGW64"
+      process.env.PSModulePath = "C:\\Program Files\\PowerShell\\Modules"
+      Object.defineProperty(process, "platform", { value: "win32" })
+
+      const result = detectShellType()
+
+      expect(result).toBe("unix")
+    })
+
+    test("#given MSYSTEM set to MSYS without SHELL #when detectShellType is called #then returns unix", () => {
+      delete process.env.SHELL
+      process.env.MSYSTEM = "MSYS"
+      process.env.PSModulePath = "C:\\Program Files\\PowerShell\\Modules"
+      Object.defineProperty(process, "platform", { value: "win32" })
+
+      const result = detectShellType()
+
+      expect(result).toBe("unix")
+    })
   })
 
   describe("shellEscape", () => {