Fixes CODAGT-447.
Alternative implementation of https://github.com/coder/coder/pull/26212
and https://github.com/coder/coder/pull/25978
- Adds up to the first 1000 characters of `README.md` (with leading
frontmatter stripped) to `chattool.list_templates` output
- Adds up to 800 characters of `README.md` to `chattool.read_template`.
**Note:** skipping `toolsdk` versions to keep scope small.
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