fix(site): make links in chat prompts clickable - #28187

Summary

URLs typed into Agents chat prompts rendered as dead plain text. This makes bare http(s) URLs in sent prompt bubbles clickable: they render as safe anchors (target="_blank", rel="noopener noreferrer") and honor the chat's urlTransform, so localhost URLs rewrite to port-forward links the same way assistant responses do. Assistant responses already render clickable links via Streamdown and are unchanged. The ticket's cmd/ctrl-press suggestion was not needed: sent bubbles have no competing click interaction (copy/edit are separate buttons).

Closes CODAGT-478.

Changes

• New linkify.ts (splitTextForLinks): plain-text URL segmentation built on linkifyjs (4.3.3, MIT), with a validate option requiring an explicit http(s) scheme so bare domains, emails, and filenames with TLD-like extensions (README.md, deploy.sh) are not linkified. Detection is best effort: known linkifyjs tokenizer limitations (trailing Markdown emphasis delimiters kept in the URL, no IPv6 literal hosts, URLs adjacent to ANSI escapes not detected) are accepted and documented in the unit tests.
• New LinkifiedText component: renders text segments with inline anchors and applies the optional urlTransform to hrefs while keeping the visible URL text.
• UserMessageContent renders prompt text (inline blocks and the plain-markdown fallback) through LinkifiedText; ConversationTimeline passes urlTransform down.
• Storybook: new UserPromptWithLinks story (href, target, rel, localhost rewrite, and real click activation with intercepted navigation) and a link assertion in UserMessageWithInlineFileRef covering linkification next to an inline file chip.

Testing

• 17 unit tests for splitTextForLinks; whole-file Storybook run for ConversationTimeline.stories.tsx (63 tests).
• pnpm -C site check and pnpm -C site lint:types pass.
• Remote dogfood UAT on dev.coder.com passed: prompt links clickable and open in a new tab, trailing punctuation excluded, multi-line prompts preserved, copy/edit interactions unaffected, response links unchanged.

Mux created this PR on Mike's behalf.