Semantic code search across multiple repos

### Describe the feature or problem you'd like to solve

Implement org-wide semantic search as an MCP server or native command in `copilot` CLI

### Proposed solution

Earlier this year, Github announced the general availability of [semantic search](https://github.blog/changelog/2025-03-12-instant-semantic-code-search-indexing-now-generally-available-for-github-copilot) in Copilot. From testing I can confirm that indexing works on individual repos within my org. If I go that repo's page and open Copilot Chat, I am able to make very fast semantic queries for code patterns within that repo. Cool.

However there a few limitations in its current form:
1. User must open Copilot Chat on a repo's homepage before indexing can begin
2. Searches are confined to 1 repo. If I want to search across N repos, it will require N separate search requests
3. Copilot Chat doesn't even know which repos are indexed anyway, because it has no "list available indices" command

Perhaps I am missing something, but altogether this doesn't provide a lot of utility. Because for a repo to even get indexed, it's a good indication that AI-happy developers have already been working with it, and at a minimum it likely contains obvious AI-friendly documentation that will allow an agent to quickly find what it's looking for. So semantic search might save some tokens but it doesn't really do anything `find` couldn't do.

What I really want is something like `Please summarize our existing approaches to rate limiting` or `Which of our NextJS apps are still on version 12?` and it would search every repo in our organization for matching patterns.

Having this ability would be extremely powerful, and being able to do it in the new `copilot` CLI even more so. It could even be the "killer feature" that accelerates CLI adoption, and you are in a unique position to provide it (since you can already see people's code)

### Example prompts or workflows

_No response_

### Additional context

_No response_