chore: add no-emdash/endash rule to agent instructions and CI lint (#24375)

Add a lint check that prevents introduction of Unicode emdash (U+2014)
and endash (U+2013) characters. These are almost exclusively introduced
by AI agents and conflict with the project writing style.

The lint script (scripts/check_emdash.sh) checks only added lines in
the current diff by default, so existing violations do not block CI.
Pass --all to scan the entire repo for auditing.

Agent instructions in AGENTS.md, site/AGENTS.md, and the docs style
guide now explicitly ban emdash, endash, and " -- " as punctuation,
with guidance to use commas, semicolons, or periods instead.

.claude/docs/DOCS_STYLE_GUIDE.md

@@ -150,6 +150,13 @@ Then ask: "Could you provide a screenshot of the Template Insights page? I've ad
   - Inline: `` `coder server` ``
   - Blocks: Use triple backticks with language identifier
 
+### Punctuation
+
+- Do not use emdash (U+2014), endash (U+2013), or ` -- ` as punctuation
+  in code, comments, string literals, or documentation. Use commas,
+  semicolons, or periods instead. Restructure the sentence if needed.
+  For numeric ranges, use a plain hyphen (e.g., `0-100`).
+
 ### Instructions
 
 - **Numbered lists** for sequential steps