Add the missing `-u` short-form alias for the `--update` option on the
`lsp add` command, aligning the implementation with the specification
(spec line 8645: `agents lsp add [--update|-u] (--config|-c) <FILE>`).
The Typer option declaration in `lsp.py` now includes `"-u"` alongside
`"--update"`. A new Behave scenario ("Add LSP server with -u short form
overwrites existing") and its step definition verify the short form works
end-to-end through the CLI runner.
No conflicts exist — `-u` was not claimed by any other option on
`lsp add` (existing short forms: `-c` for `--config`, `-f` for
`--format`).
ISSUES CLOSED: #912
brent.edwards