QONTO-WP-0003-T03: MCP client auth + shared multi-harness config snippet

Gate /mcp with a shared-secret bearer token (QONTO_ASSISTANT_MCP_TOKEN,
mcp_auth.py::BearerTokenAuthMiddleware, constant-time compare, REST
untouched) since no OIDC issuer exists in this fleet yet -- pointing
FastMCP's OAuth Protected Resource flow at a non-existent issuer would be
worse than not having it. This token is a service credential, never a bank
credential; per-actor identity stays the existing X-Actor-* convention.

Add docs/mcp-integration.md: tool catalog, the two-layer auth model (workload
auth today vs. deferred OIDC target), and one shared {"mcpServers": {...}}
client config snippet (url + headers) usable across Claude Code, Claude
Desktop, Cursor, and Codex/Grok-style harnesses.

Verified live using only that snippet: unauthenticated and wrong-token
requests get 401 before reaching any tool; a request built from the
snippet's URL + headers lists tools and calls qonto_org_summary
successfully against the fixture-backed server.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/qonto_assistant/config.py

@@ -32,6 +32,7 @@ class Settings:
     openbao_path: str
     openbao_command: str
     openbao_timeout_seconds: float
+    mcp_auth_token: str | None
     host: str
     port: int
 
@@ -69,6 +70,7 @@ class Settings:
             openbao_path=os.getenv("QONTO_OPENBAO_PATH", "tenants/binky/qonto-api"),
             openbao_command=os.getenv("QONTO_OPENBAO_COMMAND", "bao"),
             openbao_timeout_seconds=float(os.getenv("QONTO_OPENBAO_TIMEOUT_SECONDS", "5")),
+            mcp_auth_token=os.getenv("QONTO_ASSISTANT_MCP_TOKEN") or None,
             host=os.getenv("QONTO_ASSISTANT_HOST", "127.0.0.1"),
             port=int(os.getenv("QONTO_ASSISTANT_PORT", "8080")),
         )