feat(extensions,presets): authenticate GitHub-hosted catalog and download requests with GITHUB_TOKEN/GH_TOKEN (#2331)

* feat(extensions,presets): authenticate GitHub-hosted catalog and download requests with GITHUB_TOKEN/GH_TOKEN

Squashed from #2087 (original author: @anasseth).

Adds GitHub-token authentication to extension and preset catalog fetching
and ZIP downloads so private GitHub repos work when GITHUB_TOKEN/GH_TOKEN
is set, while preventing credential leakage to non-GitHub hosts.

- Introduces shared _github_http module with build_github_request() and
  open_github_url() helpers
- Routes ExtensionCatalog and PresetCatalog network calls through
  GitHub-auth-aware opener
- Adds comprehensive unit/integration tests for auth header behavior
- Updates user docs for both extensions and presets

Co-authored-by: anasseth <16745089+anasseth@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(auth): address review feedback from #2087

- Fix redirect handler to preserve Authorization on GitHub-to-GitHub
  redirects (e.g. github.com → codeload.github.com). The previous
  implementation relied on super().redirect_request() which strips
  auth on cross-host redirects, breaking private repo archive downloads.
- Add codeload.github.com to documented host lists in both
  EXTENSION-USER-GUIDE.md and presets/README.md
- Add redirect auth-preservation and auth-stripping tests

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

* fix(auth): use Bearer scheme instead of token for consistency

Aligns with the rest of the codebase (e.g. __init__.py:1721) and
GitHub's current API guidance. Updates all test assertions accordingly.

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

* fix: address second round of Copilot review feedback

- Fix docstring to say Bearer instead of token (matches implementation)
- Remove unused imports/fixtures from redirect tests (GITHUB_HOSTS,
  MagicMock, temp_dir, monkeypatch)
- Replace __import__('io').BytesIO() with normal import io pattern
  in test_presets.py

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

---------

Co-authored-by: anasseth <16745089+anasseth@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/EXTENSION-USER-GUIDE.md

@@ -423,7 +423,7 @@ In addition to extension-specific environment variables (`SPECKIT_{EXT_ID}_*`),
 | Variable | Description | Default |
 |----------|-------------|---------|
 | `SPECKIT_CATALOG_URL`       | Override the full catalog stack with a single URL (backward compat) | Built-in default stack |
-| `GH_TOKEN` / `GITHUB_TOKEN` | GitHub API token for downloads     | None                  |
+| `GH_TOKEN` / `GITHUB_TOKEN` | GitHub token for authenticated requests to GitHub-hosted URLs (`raw.githubusercontent.com`, `github.com`, `api.github.com`, `codeload.github.com`). Required when your catalog JSON or extension ZIPs are hosted in a private GitHub repository. | None |
 
 #### Example: Using a custom catalog for testing
 
@@ -435,6 +435,21 @@ export SPECKIT_CATALOG_URL="http://localhost:8000/catalog.json"
 export SPECKIT_CATALOG_URL="https://example.com/staging/catalog.json"
 ```
 
+#### Example: Using a private GitHub-hosted catalog
+
+```bash
+# Authenticate with a token (gh CLI, PAT, or GITHUB_TOKEN in CI)
+export GITHUB_TOKEN=$(gh auth token)
+
+# Search a private catalog added via `specify extension catalog add`
+specify extension search jira
+
+# Install from a private catalog
+specify extension add jira-sync
+```
+
+The token is attached automatically to requests targeting GitHub domains. Non-GitHub catalog URLs are always fetched without credentials.
+
 ---
 
 ## Extension Catalogs