Quote exec command arguments

[cookesan]

Summary

• quote multiple exec arguments when Kamal collapses them into a remote shell command
• keep the existing single-string command path unchanged
• add regression coverage for an argument containing spaces

Fixes #1651

Testing

• bin/test test/utils_test.rb test/cli/app_test.rb
• bin/test test/utils_test.rb test/cli/app_test.rb test/cli/accessory_test.rb test/cli/server_test.rb
• bin/test test/commands/app_test.rb test/commands/accessory_test.rb test/utils_test.rb
• bundle exec rubocop --parallel
• bin/test (local full-suite run: 857 runs, 3098 assertions, 3 failures, 3 errors; the isolated failures in test/commands/builder_test.rb:271 and test/commands/hook_test.rb:36 reproduce on clean upstream/main in this environment)

[Copilot]

Pull request overview

This PR fixes argument quoting for kamal * exec when multiple CLI arguments are collapsed into a single remote shell command string, while preserving the existing behavior for single-string commands (e.g., "ruby -v").

Changes:

• Update Kamal::Utils.join_commands to return the original string for single-argument commands and to shell-escape/join multi-argument commands via Shellwords.join.
• Add unit coverage for join_commands to ensure single-string commands remain unchanged and multi-arg commands preserve spaced arguments.
• Add a CLI regression test ensuring kamal app exec keeps a spaced argument intact in the emitted docker command.

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Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/kamal/utils.rb	Implements Shellwords-based joining for multi-argument exec commands while preserving the single-string path.
test/utils_test.rb	Adds focused unit tests for join_commands behavior (single string vs. multiple args).
test/cli/app_test.rb	Adds a regression test asserting spaced exec arguments remain intact in output.

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