fix: inconsistent 404 handling in GitHub API functions

[BryanFRD]

Describe the bug
In src/tools/github.ts, the two main fetch functions handle 404 differently:

• fetchRepoFile — returns null on 404 (graceful)
• fetchMatchingTags — throws GitHub API error: 404 on 404

When a package name doesn't match any tags, fetchMatchingTags should return an empty array instead of throwing, since "no matching tags" is a valid result (not an error).

Package
Which package is affected? [ ] server [x] tools [ ] docs

To Reproduce
Not provided in the original issue — to be filled in.

Expected behavior
Proposed fix:

Add 404 handling in fetchMatchingTags (around lines 106-109):

if (res.status === 404) return [];

Environment

• Package version: main
• Browser/OS (if frontend): n/a