docs(auth): document fine-grained PAT permission for org publishing (+ pagination-cap test) (#1435)

Follow-up to #1383, surfaced while validating that change end-to-end
against staging.

## Why

#1383 hardened org-namespace publishing to require Owner (`admin`) role,
read via `GET /user/memberships/orgs`. Validating on staging revealed a
gap: the docs only mention the classic `read:org` scope, but a
**fine-grained PAT** (GitHub's recommended token type) needs a
*different* permission to expose org membership. Without it, GitHub
returns no org memberships for the token, so the exchange silently
grants only the personal namespace — with no hint why org publishing
"doesn't work."

This is **not** a code bug (the behavior is fail-closed and correct
given GitHub's response); it's a documentation/UX gap.

## What

- **Docs** (`authentication.mdx`, `github-actions.mdx`): spell out both
token types —
  - Classic PAT → `read:org` scope
- Fine-grained PAT → **Organization permissions → Members → Read-only**
(the equivalent), noting a fine-grained PAT is bound to a single
resource owner.
  - Also clarified `read:user` is not required.
- **Test** (`github_at_test.go`): cover the `maxOrgMembershipPages`
pagination cap — when every page is full, the exchange **fails closed**
(no token) rather than returning a possibly-truncated org grant. This
path was previously untested.
- **Comment** (`github_at.go`): qualify the `X-GitHub-SSO` 403 check as
best-effort — we have not confirmed the memberships *list* endpoint
emits that header on SSO enforcement (it may just omit the org). The
check is safe either way, but the comment no longer overstates it as a
guarantee.

## Verification (live, against staging)

Confirmed by exchanging real tokens for `rdimitrov` on
`staging.registry.modelcontextprotocol.io`:

- **Classic `read:org` PAT** across 16 real org memberships → the JWT
granted exactly the 8 **admin** orgs + personal namespace, and **zero**
of the 8 **member** orgs (incl. `stacklok`, `modelcontextprotocol`) —
the core #1383 behavior, working.
- **Fine-grained PAT**: without Members:read → personal namespace only
(fail-closed); after adding Members:read → the org (Owner) namespace is
granted. This is what the new docs describe.

No runtime behavior change here beyond the added test; the docs and
comment are the substance.

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

internal/api/handlers/v0/auth/github_at.go

@@ -255,10 +255,15 @@ func (h *GitHubHandler) fetchOrgMembershipsPage(ctx context.Context, token strin
 		if resp.Header.Get("X-RateLimit-Remaining") == "0" || resp.Header.Get("Retry-After") != "" {
 			return nil, fmt.Errorf("GitHub API rate limit exceeded while listing org memberships (status 403): %s", readErrorBody(resp.Body))
 		}
-		// A SAML/SSO-enforced org returns 403 with an X-GitHub-SSO header when the
-		// token has not been SSO-authorized. That is an Owner being blocked, not a
-		// missing scope, so fail closed rather than silently dropping the org grant
-		// (which would degrade a legitimate Owner to personal-only with no signal).
+		// Best-effort defense: GitHub uses the X-GitHub-SSO header to signal that a
+		// resource is behind SAML/SSO the token hasn't been authorized for. If we see
+		// it, treat that as an Owner being blocked (not a missing scope) and fail
+		// closed rather than silently degrading them to personal-only. NOTE: we have
+		// not confirmed that the memberships *list* endpoint emits this header on SSO
+		// enforcement (it may instead just omit the SSO-protected org); this check is
+		// safe either way — if the header is absent we fall through to the same
+		// graceful degrade as before — but it is not a proven guarantee that every
+		// SSO-blocked Owner is caught here.
 		if resp.Header.Get("X-GitHub-SSO") != "" {
 			return nil, fmt.Errorf("GitHub org memberships require SSO authorization for this token (status 403): %s", readErrorBody(resp.Body))
 		}