feat: command-free build

[vitallium]

Zed currently has a macOS spawn bug bare executables are resolved before Command::envs is applied. So Command::new("ruby") can pick Ruby from Zed’s own process PATH, even though the child process receives the right project PATH.

That breaks Ruby language servers in projects using mise, direnv, asdf, or rbenv. The server can start under the wrong Ruby, with the wrong gems.

Resolving Ruby to an absolute path inside the extension is not a good fix. Those paths are dynamic and do not fit the extension manifest process:exec capability model.

This PR avoids the broken Command API path for language server startup by making the default build command-free (previous behavior).

Command-free launch order:

1. Use lsp.<server>.binary.path, if configured.
2. If use_bundler is true, run bundle exec <server>.
3. Otherwise resolve <server> with worktree.which.
4. Error if nothing can be launched.

use_bundler defaults

Language server	Executable	Default use_bundler
Ruby LSP	ruby-lsp	true
RuboCop	rubocop	true
Solargraph	solargraph	true
Sorbet	srb	true
Steep	steep	true
Fuzzy Ruby Server	fuzzy	N/A, custom resolver
Kanayago	kanayago	false
Herb	herb	true

Kanayago defaults to false because bundle exec kanayago fails when the gem is not in the project bundle. Once that process is launched, the extension cannot fall back to a direct executable.