Web recognises Build mentions with the shared rules #110

Part of #78. Fixes a should-fix finding from #78's Spec review.

Problem

Web and mobile recognise chat mentions with different code. Mobile uses findBuildMentions from packages/shared/src/buildLinks.ts, which skips raw HTML <code> and <a>. Web (apps/web/src/components/buildItem/buildMentionMarkdown.ts) runs BUILD_MENTION_PATTERN over mdast text nodes. With rehype-raw on, the text inside <code>#1000132</code> is an ordinary text node, so web shows a live chip inside code. #78 says mentions inside code stay as written.

Scope

Make web follow the same rules as mobile, so buildLinks.ts's claim that both clients recognise mentions with these rules is true. Raw HTML code and links must leave mentions as written. Escaped \# is out of scope: the markdown parser drops the backslash before the text reaches either recogniser's rules on web, and HQ never writes it.

Classification

Non-ui (recognition logic; no visual change).

Acceptance

On web, <code>#1000132</code> and <a href="...">#1000132</a> in a message stay as written. Fenced and inline code, #78 and links still behave as before. Tested at the web markdown pipeline level.