docs: fix broken link for chat model role in reference.mdx

[kbpr21]

Description

Fixes a broken markdown link in docs/reference.mdx. The promptTemplates
bullet listed [chat]() with an empty URL, while the sibling links (edit,
apply, autocomplete) all pointed to their respective docs pages. This
corrects the dead link.

[chat]() → [chat](/customize/model-roles/chat)

Note: the sibling links use a #prompt-templating anchor, but chat.mdx has
no such heading, so this links to the page root instead of a non-existent
anchor.

AI Code Review

• Team members only: AI review runs automatically when PR is opened or marked ready for review
• Team members can also trigger a review by commenting @continue-review

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

N/A — documentation-only change (single link fix), no UI or behavior affected.

Tests

N/A — documentation-only change. MDX files are excluded from Prettier via
.prettierignore, and no code paths or tests are affected.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[kbpr21]

I have read the CLA Document and I hereby sign the CLA