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/features/claude-code-session-state/state.ts

@@ -52,7 +52,19 @@ export function resolveRegisteredAgentName(name: string | undefined): string | u
   }
 
   const normalizedName = normalizeRegisteredAgentName(name)
-  return registeredAgentAliases.get(normalizedName) ?? normalizeStoredAgentName(name)
+  const directMatch = registeredAgentAliases.get(normalizedName)
+  if (directMatch !== undefined) return directMatch
+
+  // Resolve legacy/capitalized agent names (e.g. "Sisyphus (Ultraworker)")
+  // to their config key, then look up the registered alias for that key.
+  const configKey = getAgentConfigKey(name)
+  const normalizedConfigKey = normalizeRegisteredAgentName(configKey)
+  if (normalizedConfigKey !== normalizedName) {
+    const aliasMatch = registeredAgentAliases.get(normalizedConfigKey)
+    if (aliasMatch !== undefined) return aliasMatch
+  }
+
+  return normalizeStoredAgentName(name)
 }
 
 /** @internal For testing only */