feat: add session transfer commands

[zcutech]

Summary

• add cac env sessions commands to list, copy, and move Claude Code session history across environments
• support project-level and single-session transfers with overwrite protection
• install a cac-session-transfer Claude Code skill for newly created environments
• document session migration in English and Chinese command references
• fix timer handling on platforms where date +%s%N is unsupported

Verification

• bash build.sh
• bash -n cac
• bash -n src/cmd_env.sh
• bash -n src/templates.sh
• bash -n src/utils.sh
• verified session list/copy/move behavior with a temporary HOME