feat: add a command to update prompt templates

[nonz250]

Summary

Add a command that refreshes the prompt templates in ~/.sabori-flow/ from the ones bundled with the installed version, without going through the whole init flow.

Problem

Prompt templates are only copied during init. loadTemplate() in src/worker/prompt.ts always prefers the user directory, so template improvements shipped in a new version never reach an existing installation - the user directory shadows them permanently.

Re-running init is the only way to refresh them today, and it also walks the entire repository and execution setup and offers to overwrite config.yml. Its per-file overwrite prompt defaults to "no", shows no diff, and takes no backup, so the choice is between never updating and overwriting blind.

Confirmed on a live installation: the installed templates are byte-identical to the bundled ones, so nothing was ever customised - yet there is still no way to pull in a newer version.

Scope

• New command (for example sabori-flow update-prompts).
• Report which templates differ from the bundled version and which are already up to date.
• Per file, let the user keep or overwrite, with the option to see the diff before deciding.
• Back up the current file before overwriting.
• Never touch config.yml or the launchd registration.

Considerations

• Share the copy logic with copyPromptTemplates() in src/commands/init.ts rather than duplicating it.
• Depends on fix: resolve prompt templates by the configured language #110: the on-disk layout of ~/.sabori-flow/prompts/ becomes language-aware, and this command has to operate on the new layout.