Merge pull request #679 from shenhao-stu/fix/update-prompt-overlay-merge

fix(slash_cmds): merge additive-overlay files (e.g. .gitignore) on /update

fix: limit parallel max_workers to 3

feat: generic extra_sys_prompts slot replaces hardcoded commit_signat…

…ure module

feat: add Tauri desktop shell + desktop_bridge (url mode, no-console)

refactor: unified mykeys dict, support mykey.py and mykey.json dual f…

…ormat