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- CLAUDE.md: Removed outdated "Tier 1" references, added rejection note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>

tests/manual/test_token_exchange.py

@@ -83,6 +83,16 @@ async def main():
     logger.info("")
 
     # Step 3: Get service account token
+    # ⚠️ WARNING: Service account tokens MUST NOT be used directly with Nextcloud APIs!
+    # Using this token directly violates OAuth "act on-behalf-of" principles:
+    # - Creates Nextcloud user: service-account-{client_id}
+    # - Breaks audit trail (actions not attributable to real user)
+    # - Creates stateful server identity in Nextcloud
+    #
+    # VALID USE: ONLY as subject_token for RFC 8693 token exchange (Step 4 below)
+    # INVALID USE: Direct API access (see ADR-002 "Will Not Implement" section)
+    #
+    # If you need background operations without token exchange support, use BasicAuth mode.
     logger.info("Step 3: Requesting service account token (client_credentials)...")
     try:
         service_token_response = await oauth_client.get_service_account_token(