kamal-*: support git-style external commands in .kamal/bin and on PATH

[jeremy]

Summary

• Resolve unknown commands as external executables before falling through to aliases/errors — same pattern as git-foo on PATH
• Lookup order: .kamal/bin/<name> (project-local), then kamal-<name> on PATH
• Builtins (including prefix matches like dep → deploy) are never shadowed
• External commands bypass config loading, so they work even with broken/missing deploy.yml
• Guards against directories, non-executable files, path traversal, and flags

Test plan

• Local .kamal/bin/foo → exec called with correct path
• PATH kamal-bar → exec called
• Local takes priority over PATH when both exist
• Builtins not shadowed (version with .kamal/bin/version present)
• Prefix builtins not shadowed (ver with .kamal/bin/ver present)
• Arguments passed through to external command
• Non-executable file skipped
• Directory in .kamal/bin skipped
• Directory on PATH skipped
• Path traversal (../foo) rejected
• Existing test suite passes (55 runs, 376 assertions)

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[Copilot]

Pull request overview

This PR adds support for external commands that can extend Kamal's functionality, following a plugin pattern similar to Git's git-foo extension model. External commands are resolved when no builtin command matches, looking first in .kamal/bin/<name> (project-local), then for kamal-<name> on PATH. These external commands bypass config loading, allowing them to work even with broken or missing configuration files, while builtin commands (including prefix matches) are never shadowed.

Changes:

• Added dispatch override in Kamal::Cli::Main to resolve and execute external commands
• Implemented resolve_external_command method with security guards against path traversal, directories, non-executable files, and flags
• Added comprehensive test coverage for external command resolution, priority, and security scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/kamal/cli/main.rb	Implements dispatch override and external command resolution with security checks
test/cli/main_test.rb	Adds 11 comprehensive tests covering external command behavior and security

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