Link #123 issue and pull request references in chat (#8636)

* feat(app): link #123 issue and pull request references in Markdown

Agents cite issues and pull requests by number; reading one meant looking
up the URL by hand. Markdown in a workspace now links forge shorthand
against the workspace's origin remote: `#N`, GitLab and Gitea `!N`, and
cross-repository `owner/repo#N` on the same host.

Each forge's URL grammar declares which sigil maps to which path, so the
syntax stays per forge: GitHub has no `!`, and its `#` targets /issues/,
which GitHub redirects to the pull request. Recognition goes through the
Markdown engine's text link regex so code spans, fences and authored links
stay plain without a second Markdown parse while streaming. The reference
alternative sits before path recognition so `group/project#12` is not read
as a file path.

* fix(app): keep http:// for self-hosted forge web links

A self-hosted forge cloned over http:// serves its web UI over http on
the same origin. Forcing https made reference, blob and tree links fail
against an HTTP-only port. Cloud hosts and SSH remotes stay on https.

packages/app/src/git/client-forge-module.ts

@@ -41,8 +41,15 @@ export interface ForgeReferencePath {
   kind: "change_request" | "issue";
   /** Path between the repository path and the numeric local id. */
   infix: string;
+  /**
+   * Shorthand marker the forge links in prose, e.g. GitHub `#12`, GitLab `!12`.
+   * Absent when the forge has no shorthand for this kind.
+   */
+  sigil?: ForgeReferenceSigil;
 }
 
+export type ForgeReferenceSigil = "#" | "!";
+
 /** Line/range anchor shared by every GitHub-family forge (github, gitea, forgejo, codeberg). */
 export function GITHUB_LINE_ANCHOR(start: number, end?: number): string {
   return end && end > start ? `#L${start}-L${end}` : `#L${start}`;
@@ -54,8 +61,8 @@ export const GITEA_FAMILY_URL_GRAMMAR: ForgeUrlGrammar = {
   blobInfix: "/src/branch/",
   lineAnchor: GITHUB_LINE_ANCHOR,
   referencePaths: [
-    { kind: "change_request", infix: "/pulls/" },
-    { kind: "issue", infix: "/issues/" },
+    { kind: "change_request", infix: "/pulls/", sigil: "!" },
+    { kind: "issue", infix: "/issues/", sigil: "#" },
   ],
 };