refactor: remove Smithery deployment mode

Smithery is no longer a supported deployment mode. Remove all Smithery-specific
code paths, middleware, configuration, and tests. This simplifies the codebase
by eliminating DeploymentMode enum, SmitheryConfigMiddleware, session config
context variables, and the smithery_main entrypoint.

Files deleted: Dockerfile.smithery, smithery.yaml, smithery_main.py
ADR-016 retained with deprecated status for historical reference.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

env.sample

@@ -4,7 +4,7 @@
 # Optional: Explicitly declare deployment mode (ADR-021)
 # If not set, mode is auto-detected from other settings
 # Valid values: single_user_basic, multi_user_basic, oauth_single_audience,
-#               oauth_token_exchange, smithery
+#               oauth_token_exchange
 #
 # Recommendation: Set this for clarity and to catch configuration errors early
 #MCP_DEPLOYMENT_MODE=oauth_single_audience
@@ -119,16 +119,6 @@ NEXTCLOUD_PASSWORD=
 # Optional features (semantic search, document processing):
 # See "Optional Features" section below
 
-# ============================================
-# SMITHERY STATELESS MODE
-# ============================================
-# Stateless multi-tenant deployment for Smithery platform
-# Configuration comes from session URL parameters
-# No persistent storage, no OAuth, no vector sync
-#
-# Required: None (all config from session URL)
-# This mode is activated automatically when deployed to Smithery
-
 # ============================================
 # OPTIONAL FEATURES (All Deployment Modes)
 # ============================================