chore(gitignore): anchor binary names to repo root (#2522)

The bare `github-mcp-server`, `mcpcurl`, and `e2e.test` rules matched
those names anywhere in the tree, which silently ignored new files
created under `cmd/github-mcp-server/` (the rule treats the directory
component as a match). The intent was to ignore the binaries produced
by `go build` at repo root, so anchor each rule with a leading slash.

The existing `cmd/github-mcp-server/github-mcp-server` rule on line 2
continues to ignore the binary when built inside the cmd directory.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitignore

@@ -17,9 +17,9 @@ bin/
 .DS_Store
 
 # binary
-github-mcp-server
-mcpcurl
-e2e.test
+/github-mcp-server
+/mcpcurl
+/e2e.test
 
 .history
 conformance-report/