Add post-switch hook support

[grauschnabel]

Summary

• Add configurable post_switch hook in ~/.ccswitch/config.yaml that runs after every worktree switch
• Hook is triggered by create, checkout, switch, and list commands
• Shell integration (Bash + Zsh) extracts and evaluates hook commands via #post-hook: markers
• Refactor worktree base path logic into shared getWorktreeBasePath() helper

Example config

hooks:
  post_switch: "claude"

Test plan

• Configure post_switch hook and verify it runs after ccswitch create
• Verify hook runs after ccswitch switch, checkout, and list
• Verify no hook runs when config is absent
• Test with both Bash and Zsh shell integration

🤖 Generated with Claude Code

[grauschnabel]

Note: The refactoring in internal/session/manager.go (extracting getWorktreeBasePath()) is a separate cleanup — it deduplicates the worktree base path logic that was inline in CreateSession and repeated in GetSessionPath. No behavioral change, just reducing duplication. Could have been a separate commit.