Emit bash language token for CLI usage fences

[allister-beamable]

What

DocService.GetUsage (cli/cli/Services/Docs/DocService.cs) wraps each command's usage template in a ```shell fence. This changes it to ```bash.

Why

The docs site standardizes on bash as the single language token for typed shell commands (the hand-written CLI guides were just normalized shell/sh → bash in beamable/docs). The generated CLI command reference is regenerated from this code, so its ~200-per-branch usage fences can only be standardized here — editing the generated Markdown in the docs repo would be overwritten on the next regen.

shell, sh, and bash all resolve to the same Pygments Bash lexer, so rendering is unchanged; this is a source-consistency fix that lets the whole docs site use one token per language.

Scope

One-line string change; the usage block is a typed command template (beam <command> [options]), so bash is the correct token. No behavior change beyond the emitted token.

🤖 Generated with Claude Code

[allister-beamable]

Closing — the docs will standardize on shell to match this generator's existing token, rather than changing the generator to bash. No SDK change needed.