Composer #: also mention issues and discussions

[Huijiro]

Problem

Typing # in the composer only finds pull requests. Agents often need issue or discussion context (bug report, design thread, RFC) and users already think of #N as the shared GitHub number space. Today that means pasting a URL or opening the host manually.

Proposed direction / scope

Extend the existing # autocomplete so one mixed menu can return:

• GitHub: pull requests, issues, and discussions
• GitLab: merge requests and issues (shared IID collisions show both)
• Other hosts: keep today’s PR-only behavior

Selecting an issue or discussion inserts a composer context chip (same attach path as PR chips). Keep the current PR attach / link-to-thread behavior unchanged.

Out of scope for this change: formal draft kinds beyond the existing context chip path, Bitbucket/Forgejo issue search, and changing how linked PRs attach to threads.

Ask

Maintainers: please approve or redirect this scope before deeper review of the implementation.

Implementation draft: #27902

[Huijiro]

Implementation PR (updated for CONTRIBUTING): #27902

Still need an explicit maintainer approval of the direction/scope described above before this should go through deeper review as an outside feature contribution.