feat(mcp): allowlist Grok's connector callback and document the Grok path (#2158)

* feat(mcp): allowlist Grok's connector callback and document the Grok path

Grok custom connectors self-register through /api/mcp-oauth/register with
redirect_uri https://grok.com/connectors-oauth-exchange-code/, which the
built-in allowlist rejected with invalid_redirect_uri before consent. Add
the callback as an exact-path BUILT_IN_PATTERNS entry (trailing slash
optional, no prefix) with provider 'grok', named "Grok (xAI)" on the
consent page. Tests: accept, foreign-host and other-path rejection,
provider mapping, and a register route test for the Grok DCR shape.

Surface Grok next to ChatGPT: a "Using Grok?" side door on the onboarding
Claude step (one side door open at a time, telemetry step grok), a Grok row
under "Other clients" in the API & MCP settings tab using ?client=grok, and
sv/en strings for both. Docs: mcp-server rule, ARCHITECTURE, README,
registry entry (install section), DECISIONS.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGbspj3hiNqvqTWZqdwysa
Signed-off-by: Emil <emilmattsson14@gmail.com>

* fix(mcp): cite X Corp's published Grok callback, test the consent label

Review pass on #2158: the allowlist comment and DECISIONS entry claimed
xAI publishes no callback and the value came from a live observation; X
Corp lists https://grok.com/connectors-oauth-exchange-code/ as the "Grok
(web)" redirect URL at docs.x.com/x-ads-api/mcp, and grok.com serves the
path itself (slash form 308s to no-slash on the same origin). Reworded
both to cite that. Adds the consent-page test for "Grok (xAI)" next to
the ChatGPT one and a JSDoc on the onboarding side-door toggle.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGbspj3hiNqvqTWZqdwysa
Signed-off-by: Emil <emilmattsson14@gmail.com>

---------

Signed-off-by: Emil <emilmattsson14@gmail.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

.claude/rules/mcp-server.md

@@ -11,7 +11,7 @@ Accounted exposes its bookkeeping engine as an MCP server for Claude Desktop/Cod
 
 **MCP extension** (`extensions/general/mcp-server/`): 150+ tools (count `name: 'gnubok_` in `server.ts`; docs say "150+", never an exact number, because it drifts) covering transactions, categorization, customers/suppliers, invoices, accounts, fiscal periods, reports (trial balance, GL, BS, IS, AR/supplier ledger, VAT, KPI), reconciliation, salary runs, AGI, year-end, document upload, and loadable skills. JSON-RPC 2.0. Endpoint: `/api/extensions/ext/mcp-server/mcp`.
 
-**OAuth 2.1** for Claude and ChatGPT connectors: `.well-known/oauth-protected-resource` + `.well-known/oauth-authorization-server` discovery; `/api/mcp-oauth/authorize`, `/token` (PKCE), `/register`. Stateless AES-256-GCM auth codes (`lib/auth/oauth-codes.ts`). Single-use via `oauth_used_codes`. Allowlist: `claude.ai/api/*`, `claude.com/api/*`, `chatgpt.com/connector/oauth/*`, `chatgpt.com/connector_platform_oauth_redirect`, `localhost`.
+**OAuth 2.1** for Claude, ChatGPT and Grok connectors: `.well-known/oauth-protected-resource` + `.well-known/oauth-authorization-server` discovery; `/api/mcp-oauth/authorize`, `/token` (PKCE), `/register`. Stateless AES-256-GCM auth codes (`lib/auth/oauth-codes.ts`). Single-use via `oauth_used_codes`. Allowlist: `claude.ai/api/*`, `claude.com/api/*`, `chatgpt.com/connector/oauth/*`, `chatgpt.com/connector_platform_oauth_redirect`, `grok.com/connectors-oauth-exchange-code/` (exact), `localhost`.
 
 **npm packages**: `packages/accounted-mcp` is the Accounted stdio-to-HTTP bridge for new installs. `packages/gnubok-mcp` is the permanent compatibility package for existing configurations.