feat(links): make .diff and .patch paths clickable

An agent that writes a diff for review prints a path such as
/tmp/.../scratchpad/resolution.diff, and that path rendered as plain text
because the link patterns had no diff or patch extension. The attachment
allowlist already serves both as inert text/plain, so only the two link
patterns in constants.js change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/link-provider-regex.test.ts

@@ -153,6 +153,9 @@ describe('terminal link-provider regexes (shipped source)', () => {
       '/home/a/.codeman-uploads/paste-2-cd.ogv',
       // Longer extension of a family must win over its prefix (tsx over ts).
       '/home/a/src/App.tsx',
+      // Diffs an agent writes for review; the attachment allowlist already serves them.
+      '/tmp/claude-1000/-home-a-repo/b3de6e9f/scratchpad/resolution.diff',
+      '/home/a/repo/fix.patch',
     ];
     for (const path of cases) {
       ext.lastIndex = 0;