Bare `#NNNNN` auto-linkifies against the current repo, breaking cross-repo issue/PR references

### Summary

In the CLI's rendered output, a bare `#NNNNN` is auto-linkified as an issue/PR mention resolved against the **current repository's** remote. When the number actually refers to a *different* repository, this produces an incorrect link.

### Environment

- GitHub Copilot CLI v1.0.64 (Linux)

### Example

While working in repository `owner/repoA`, referring to an upstream pull request from another project as `#205054` renders as a link to `https://github.com/owner/repoA/issues/205054` — a bogus URL (there is no such issue in `repoA`; the real reference is `other-org/other-repo#205054`).

### Expected

One of:
- Don't auto-linkify a bare `#NNNNN` unless it resolves to an existing issue/PR in the current repo, **or**
- Provide a setting to disable issue/PR auto-linkification, **or**
- Only auto-linkify qualified references (`owner/repo#NNNN`).

### Impact

Easy to hit whenever working in one repo (e.g. a fork or mirror) while citing issues/PRs from another (e.g. an upstream project). The generated links are misleading. Minor but common.

### Workarounds

Full URL (`https://github.com/org/repo/pull/NNNN`), a qualified `owner/repo#NNNN`, or wrapping the literal in backticks (`` `#NNNN` ``) so it isn't linkified.

### Note

`#` is documented in the CLI help as "mention issues and pull requests". The request is to make a bare cross-repo number either non-linkified or configurable.

No source file or test is identified in the issue. Start by locating the CLI's rendered-output handling for issue and pull-request auto-linkification, then reproduce the cross-repository `#205054` example; done means bare references no longer produce misleading current-repository links, with the selected behavior covered by tests.

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