fix(migration): run migration during install, not on first execution (v3.1.1)

Fixed:

- Migration now runs during installation across all methods (npm, bash, PowerShell)

- Guarantees ~/.ccs/shared/ populated immediately with ~/.claude/ content

- Users no longer need to run ccs command to trigger migration

Changed:

- Refactored SharedManager with _needsMigration() and _performMigration() methods

- _copyDirectory() returns {copied, skipped} stats and preserves existing files

- Shows detailed migration output: '[OK] Migrated 5 commands, 19 skills'

- Removed lazy migration from bin/ccs.js, lib/ccs, lib/ccs.ps1

Implementation:

- npm: Migration in scripts/postinstall.js

- bash: Migration in installers/install.sh (migrate_shared_data function)

- PowerShell: Migration in installers/install.ps1 (Invoke-SharedDataMigration)

- Fixed arithmetic expansion with set -e (changed ((var++)) to var=$((var + 1)))

Cross-platform parity maintained across all installation methods.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@kaitranntt/ccs",
-  "version": "3.1.0",
+  "version": "3.1.1",
   "description": "Claude Code Switch - Instant profile switching between Claude Sonnet 4.5 and GLM 4.6",
   "keywords": [
     "cli",