fix: filter integration scripts by script_type

[kailanjian]

Description

install_scripts() accepted a script_type parameter to filter which script variant to copy (.sh or .ps1), but every caller in MarkdownIntegration, TomlIntegration, SkillsIntegration, and ForgeIntegration invoked it without passing the type. The result: --script sh had no effect on the physical file copy — both update-context.sh and update-context.ps1 were always installed regardless of the flag.

The .ps1 file landed on disk but was never referenced (the generated skills and integration.json correctly used the sh variant), making it inert dead weight on Mac/Linux.

The --script flag already correctly switched three other things:

• Which shared scripts directory is copied (bash/ vs powershell/)
• Which script path gets embedded in generated skill/command markdown
• Which filename integration.json records

This PR closes the gap for the fourth: which files are physically copied by install_scripts().

Changes:

• integrations/base.py — add script_type: str | None = None param + filter logic to install_scripts(); pass script_type through in MarkdownIntegration, TomlIntegration, and SkillsIntegration setup() methods
• integrations/forge/__init__.py — same one-line fix in ForgeIntegration.setup() (overrides setup() directly, so wasn't covered by the base class change)

Testing

• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project: specify-af init --here --ai claude --script sh --force no longer generates update-context.ps1

AI Disclosure

• I did use AI assistance (describe below)

Claude Code (claude-sonnet-4-6) identified the root cause, implemented the fix, and updated tests.