docs: Reject service account tokens as OAuth authentication pattern

Service account tokens (client_credentials grant) violate OAuth "act on-behalf-of"
principles and have been moved to ADR-002's "Will Not Implement" section.

## Problem Discovery

Testing revealed that service account tokens create Nextcloud user accounts
(e.g., `service-account-nextcloud-mcp-server`) due to user_oidc's bearer
provisioning feature. This violates core OAuth principles:

- ❌ Creates stateful server identity in Nextcloud
- ❌ All actions attributed to service account, not real user
- ❌ Breaks audit trail and user attribution
- ❌ Service account becomes "admin by another name"

## Changes

### Documentation (ADR-002)
- Moved service account (old Tier 1) to "Will Not Implement" section
- Added "OAuth Act On-Behalf-Of Principle" section
- Renumbered tiers:
  - Tier 1: Impersonation (NOT IMPLEMENTED)
  - Tier 2: Delegation via token exchange (IMPLEMENTED)
- Updated status to reflect rejection of service accounts

### Code Warnings
- Added comprehensive warning to KeycloakOAuthClient.get_service_account_token()
- Clarified VALID use: only as subject_token for RFC 8693 token exchange
- Clarified INVALID use: direct API access with service account token

### Supporting Documentation
- CLAUDE.md: Removed outdated "Tier 1" references, added rejection note
- oauth-impersonation-findings.md: Added prominent update banner
- audience-validation-setup.md: Updated tier numbers, added rejection note
- tests/manual/test_token_exchange.py: Added warning comment

## Valid Patterns (ADR-002)

✅ Foreground operations: User's access token from MCP request
✅ Background operations: Token exchange (impersonation/delegation)
✅ Offline access: Refresh tokens with user consent
❌ Service accounts: Creates independent server identity (REJECTED)

## Alternative

If service account pattern is truly needed, use BasicAuth mode instead of
OAuth mode. OAuth mode MUST maintain "act on-behalf-of" semantics.

Related: c12df98 (revert of service account test)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/auth/keycloak_oauth.py

@@ -342,9 +342,27 @@ class KeycloakOAuthClient:
         """
         Get a service account token using client_credentials grant.
 
-        This requires the client to have serviceAccountsEnabled=true in Keycloak.
-        The service account token can be used for server-initiated operations
-        or as the subject_token for token exchange.
+        ⚠️ **WARNING: DO NOT USE FOR DIRECT API ACCESS IN OAUTH MODE** ⚠️
+
+        This method creates a service account user in Nextcloud which VIOLATES
+        OAuth "act on-behalf-of" principles. Using this token directly for API
+        access will:
+        - Create a Nextcloud user: `service-account-{client_id}`
+        - Attribute all actions to service account instead of real user
+        - Break audit trail and user attribution
+        - Create stateful server identity in Nextcloud
+        - Violate OAuth security model
+
+        **Valid Use Case**: ONLY as subject_token for RFC 8693 token exchange
+        (ADR-002 Tier 2) where it's immediately exchanged for a user token.
+
+        **Invalid Use Case**: Direct API access with this token (ADR-002 rejected
+        this as "Tier 1" - see docs/ADR-002-vector-sync-authentication.md).
+
+        **Alternative**: Use token exchange (impersonation/delegation) for
+        background operations, or use BasicAuth mode if truly need service account.
+
+        This requires the client to have serviceAccountsEnabled=true in provider.
 
         Args:
             scopes: Optional list of scopes to request (default: openid profile email)
@@ -359,9 +377,9 @@ class KeycloakOAuthClient:
         Raises:
             httpx.HTTPError: If token request fails
 
-        Note:
-            This is used for ADR-002 Tier 2 (Token Exchange). The service account
-            token is exchanged for user-scoped tokens via RFC 8693.
+        See Also:
+            - ADR-002 "Will Not Implement" section for detailed critique
+            - exchange_token_for_user() for proper token exchange usage
         """
         if not self.token_endpoint:
             await self.discover()