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- CLAUDE.md: Removed outdated "Tier 1" references, added rejection note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

CLAUDE.md

@@ -421,7 +421,7 @@ curl http://localhost:8888/realms/nextcloud-mcp/.well-known/openid-configuration
 # 3. Verify user_oidc provider is configured
 docker compose exec app php occ user_oidc:provider keycloak
 
-# 4. Generate encryption key for refresh token storage (optional, for ADR-002 Tier 1)
+# 4. Generate encryption key for refresh token storage (optional, for offline access)
 python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
 # Set in environment: export TOKEN_ENCRYPTION_KEY='<key>'
 
@@ -507,13 +507,15 @@ docker compose exec app curl http://keycloak:8080/realms/nextcloud-mcp/.well-kno
 ```
 
 **ADR-002 Offline Access Testing:**
-The Keycloak integration enables testing ADR-002 Tier 1 (offline access with refresh tokens):
+The Keycloak integration enables testing ADR-002's primary authentication pattern (offline access with refresh tokens):
 
 1. **Refresh token storage**: Tokens stored encrypted in SQLite (`/app/data/tokens.db`)
 2. **Token refresh**: Access tokens refreshed automatically when expired
 3. **Background workers**: Can access APIs using stored refresh tokens
 4. **No admin credentials**: All operations use user's OAuth tokens
 
+**Note**: Service account tokens (client_credentials grant) were considered but rejected as they create Nextcloud user accounts and violate OAuth "act on-behalf-of" principles. See ADR-002 "Will Not Implement" section.
+
 See `docs/ADR-002-vector-sync-authentication.md` for architectural details.
 
 **Audience Validation:**