refactor: integrate token exchange into unified get_client() pattern

Resolves the token exchange implementation gap where get_session_client()
was implemented but never used by tools. Unifies token acquisition into a
single async get_client() method that handles both pass-through and token
exchange modes transparently.

Core Changes:
- Make get_client() async and merge token exchange logic into it
- Remove scopes parameter from token exchange (Nextcloud doesn't support OAuth scopes)
- Update all 8 tool modules to use await get_client(ctx)
- Fix provisioning decorator to skip checks in BasicAuth mode

Token Acquisition Modes:
1. BasicAuth: Returns shared client (no token operations)
2. OAuth pass-through (default): Verifies and passes Flow 1 token to Nextcloud
3. OAuth token exchange (opt-in): Exchanges Flow 1 token for ephemeral token via RFC 8693

Key Architectural Clarifications:
- Progressive Consent (Flow 1/2) = Authorization architecture
- Token Exchange = Token acquisition pattern during tool execution
- Refresh tokens from Flow 2 are NEVER used for tool calls (only background jobs)
- Nextcloud scopes are "soft-scopes" enforced by MCP server, not IdP

Documentation Updates:
- ADR-004: Added comprehensive token acquisition patterns section
- CRITICAL-TOKEN-EXCHANGE-PATTERN.md: Updated to reflect implementation status
- CLAUDE.md: Updated architectural patterns with async get_client()

Testing:
- All 36 unit tests passing
- All 4 smoke tests passing (BasicAuth mode)
- Linting issues fixed (ruff)

Configuration:
ENABLE_TOKEN_EXCHANGE=false (default) - pass-through mode
ENABLE_TOKEN_EXCHANGE=true (opt-in) - token exchange mode

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

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

nextcloud_mcp_server/server/sharing.py

@@ -45,7 +45,7 @@ def configure_sharing_tools(mcp: FastMCP):
         Returns:
             JSON string with share information including share ID
         """
-        client = get_client(ctx)
+        client = await get_client(ctx)
         share_data = await client.sharing.create_share(
             path=path,
             share_with=share_with,
@@ -67,7 +67,7 @@ def configure_sharing_tools(mcp: FastMCP):
         Returns:
             JSON string confirming deletion
         """
-        client = get_client(ctx)
+        client = await get_client(ctx)
         await client.sharing.delete_share(share_id)
         return json.dumps(
             {"success": True, "message": f"Share {share_id} deleted"}, indent=2
@@ -87,7 +87,7 @@ def configure_sharing_tools(mcp: FastMCP):
         Returns:
             JSON string with share information
         """
-        client = get_client(ctx)
+        client = await get_client(ctx)
         share_data = await client.sharing.get_share(share_id)
         return json.dumps(share_data, indent=2)
 
@@ -106,7 +106,7 @@ def configure_sharing_tools(mcp: FastMCP):
         Returns:
             JSON string with list of shares
         """
-        client = get_client(ctx)
+        client = await get_client(ctx)
         shares = await client.sharing.list_shares(
             path=path, shared_with_me=shared_with_me
         )
@@ -133,7 +133,7 @@ def configure_sharing_tools(mcp: FastMCP):
         Returns:
             JSON string with updated share information
         """
-        client = get_client(ctx)
+        client = await get_client(ctx)
         share_data = await client.sharing.update_share(
             share_id=share_id, permissions=permissions
         )