fix: accept enterprise managed GitHub owner logins (#20450)

Unify owner validation across project pickers and repository overrides. Preserve EMU usernames in API and auth-status branch-prefix resolution, with regression coverage.

Co-authored-by: Neil <neil@stably.ai>

src/main/github/github-api-repository-validation.test.ts

@@ -0,0 +1,25 @@
+// Why: owner/repo overrides become authenticated REST paths, so the slug gate
+// must keep rejecting path-shaped input while accepting every real login shape —
+// including Enterprise Managed User logins, which end in `_<shortcode>`.
+import { describe, expect, it } from 'vitest'
+import { isValidGitHubApiRepository } from './github-api-repository-validation'
+
+describe('isValidGitHubApiRepository', () => {
+  it('accepts plain and Enterprise Managed User owners', () => {
+    expect(isValidGitHubApiRepository({ owner: 'acme', repo: 'orca' })).toBe(true)
+    expect(isValidGitHubApiRepository({ owner: 'octocat_acme', repo: 'level5' })).toBe(true)
+  })
+
+  it('rejects leading underscore, hyphen, dot, and path-shaped owners', () => {
+    expect(isValidGitHubApiRepository({ owner: '_acme', repo: 'orca' })).toBe(false)
+    expect(isValidGitHubApiRepository({ owner: '-acme', repo: 'orca' })).toBe(false)
+    expect(isValidGitHubApiRepository({ owner: '.acme', repo: 'orca' })).toBe(false)
+    expect(isValidGitHubApiRepository({ owner: 'a/b', repo: 'orca' })).toBe(false)
+  })
+
+  it('rejects reserved and path-shaped repos', () => {
+    expect(isValidGitHubApiRepository({ owner: 'acme', repo: '.' })).toBe(false)
+    expect(isValidGitHubApiRepository({ owner: 'acme', repo: '..' })).toBe(false)
+    expect(isValidGitHubApiRepository({ owner: 'acme', repo: 'a/b' })).toBe(false)
+  })
+})