stdin / pipe input with --lang override

Allow piping code in: `cat f.ts | snipshot - --lang ts`, enabling `git diff | snipshot`. Requires a language override since there's no extension.
- `snipshot -` reads stdin
- `--lang ` forces the Shiki language (extend `resolveLanguage()`)

- [ ] stdin reading
- [ ] `--lang` override + validation against Shiki languages

Start by locating the CLI entry point that handles the input path and the existing resolveLanguage() function. Run the documented file-input command, then try `cat f.ts | snipshot -` and `cat f.ts | snipshot - --lang ts` to observe current behavior. Done means stdin is accepted and --lang is validated against the available Shiki languages.

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