Add util for windows sub-process

[LahkLeKey]

This pull request adds a utility function to determine the correct executable name for the Copilot CLI based on the operating system. This helps ensure that subprocess calls work reliably across platforms.

Platform compatibility improvement:

• Added the _copilot_executable() function to return the appropriate Copilot CLI executable name (copilot.cmd on Windows, copilot otherwise), improving subprocess invocation reliability on different platforms.## Description

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

16 failed, 2952 passed, 3 skipped in 335.38s (0:05:35)

========================================================================================================================= short test summary info =========================================================================================================================
FAILED tests/integrations/test_cli.py::TestInitIntegrationFlag::test_shared_infra_skip_warning_displayed - AssertionError: assert 'specify integration upgrade --force' in '\x1b[33m⚠\x1b[0m 1 shared infrastructure file(s) already exist and were not updated: .specify/scripts/bash/common.sh To refresh shared infrastructure, run \x1b[36mspecify init --here --force\x1b[0m ...
FAILED tests/integrations/test_integration_subcommand.py::TestIntegrationList::test_list_shows_multi_install_safe_status - StopIteration
FAILED tests/integrations/test_integration_subcommand.py::TestIntegrationInstall::test_install_already_installed - assert 'specify integration upgrade copilot' in "\x1b[33mIntegration 'copilot' is already installed.\x1b[0m It is already the default integration. To refresh its managed files or options, run \x1b[36mspecify integration upgrade \x1b[0m \x1b[36mcopilot\x1b[0m. No ...
FAILED tests/integrations/test_integration_subcommand.py::TestIntegrationInstall::test_install_different_when_one_exists - assert 'Default integration: copilot' in "\x1b[31mError:\x1b[0m Installed integrations: copilot.\nDefault integration: \x1b[36mcopilot\x1b[0m.\nInstalling multiple integrations is only automatic when all involved \nintegrations are declared multi-install safe.\nT...
FAILED tests/integrations/test_integration_subcommand.py::TestIntegrationInstall::test_install_multi_unsafe_requires_force - assert 'retry the same install command with --force' in "\x1b[31mError:\x1b[0m Installed integrations: copilot. Default integration: \x1b[36mcopilot\x1b[0m. Installing multiple integrations is only automatic when all involved integrations are declared multi-insta...
FAILED tests/test_setup_tasks.py::test_setup_tasks_bash_core_template_resolved - AssertionError: TASKS_TEMPLATE must be an absolute path
FAILED tests/test_setup_tasks.py::test_setup_tasks_bash_override_wins - AssertionError: TASKS_TEMPLATE must be an absolute path
FAILED tests/test_setup_tasks.py::test_setup_tasks_bash_extension_wins_over_core - AssertionError: TASKS_TEMPLATE must be an absolute path
FAILED tests/test_setup_tasks.py::test_setup_tasks_bash_preset_wins_over_extension - AssertionError: TASKS_TEMPLATE must be an absolute path
FAILED tests/test_setup_tasks.py::test_setup_tasks_bash_preset_priority_order - AssertionError: TASKS_TEMPLATE must be an absolute path
FAILED tests/test_timestamp_branches.py::TestTimestampBranch::test_long_name_truncation - AssertionError: [specify] Warning: Branch name exceeded GitHub's 244-byte limit
FAILED tests/test_timestamp_branches.py::TestGetFeaturePathsSinglePrefix::test_bash_specify_feature_prefixed_resolves_by_prefix - AssertionError: assert '/tmp/pytest-...1-target-spec' == 'C:\\Users\\Z...1-target-spec'
FAILED tests/test_timestamp_branches.py::TestFeatureDirectoryResolution::test_env_var_overrides_branch_lookup - assert 'C:\\Users\\<MachineName>\\AppData\\Local\\Temp\\pytest-of-<MachineName>\\pytest-34\\test_env_var_overrides_branch_0\\my-custom-specs\\my-feature' in 'REPO_ROOT=/tmp/pytest-of-<MachineName>/pytest-34/test_env_var_overrides_branch_0\nCURRENT_BRANCH=master\nHAS_GIT=true\nFEATURE_DI...
FAILED tests/test_timestamp_branches.py::TestFeatureDirectoryResolution::test_feature_json_overrides_branch_lookup - AssertionError: assert '/tmp/pytest-...ustom-feature' == 'C:\\Users\\Z...ustom-feature'
FAILED tests/test_timestamp_branches.py::TestFeatureDirectoryResolution::test_env_var_takes_priority_over_feature_json - AssertionError: assert '/tmp/pytest-...\\env-feature' == 'C:\\Users\\Z...\\env-feature'
FAILED tests/test_timestamp_branches.py::TestFeatureDirectoryResolution::test_fallback_to_branch_lookup - AssertionError: assert '/tmp/pytest-...001-test-feat' == 'C:\\Users\\Z...001-test-feat'
========================================================================================================= 16 failed, 2952 passed, 3 skipped in 335.38s (0:05:35) ==========================================================================================================

<MachineName>@<MachineName> MINGW64 /c/Github/spec-kit (main)

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Ran into this after install when working on other project, probably handy to have

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a platform-aware helper to resolve the Copilot CLI executable name, using copilot.cmd on Windows for reliable subprocess invocation.

Changes:

• New _copilot_executable() helper returning copilot.cmd on Windows and copilot otherwise.

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[Copilot]

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.