fix(chat): add prompt tools when running prompt slash commands

[petobens]

Description

When a prompt library entry is invoked via a slash command, any tools declared
on that prompt were not being added to the active chat buffer first.

This PR fixes that by registering prompt-declared tools before the prompt
content is inserted.

Changes included:

• add prompt tools in SlashCommands.run() for prompt-library entries
• support both single tools and tool groups
• add tests covering both cases
• document the behavior in prompt-library.md

This makes slash-command usage consistent with prompt definitions that rely on
tools being available in the current chat.

AI Usage

GPT-5.4 was used mostly for tests, docs, and general boilerplate such as
docstrings.

Related Issue(s)

Screenshots

Checklist

• I've read the contributing guidelines and have adhered to them in this PR
• I confirm that this PR has been majority created by me, and not AI (unless stated in the "AI Usage" section above)
• I've run make all to ensure docs are generated, tests pass and StyLua has formatted the code
• (optional) I've added test coverage for this fix/feature
• (optional) I've updated the README and/or relevant docs pages