feat(agents): consolidate connectors under mcp_discovery; route web search through google_search

MCP consolidation:
- Route all MCP-capable connectors (Slack, Jira, Linear, ClickUp, Airtable,
  Notion, Confluence, interim Gmail/Calendar, custom MCP) through a single
  `mcp_discovery` subagent. Drive/OneDrive/Dropbox stay native to enrich the KB.
- Deprecate Discord/Teams/Luma: no viable official MCP server.

Google-only web search:
- Remove the main-agent `web_search` tool and the SearXNG platform service;
  all public web search now flows through the `google_search` subagent via task().
- Deprecate the Tavily/SearXNG/Linkup/Baidu search connectors (HTTP 410 on
  create, "Deprecated" badge); guide heavy users to the custom MCP connector.
- Remove web search from anonymous chat (pure Q&A).
- Tear SearXNG out of docker compose + install scripts; drop tavily-python
  and linkup-sdk deps and their config/env vars.

Fix:
- metrics._package_version() now swallows any metadata lookup failure. A
  malformed editable-install distribution with no `Version` field raised
  KeyError deep in importlib.metadata, and since it runs on every
  record_subagent_invoke_duration call it was crashing every task()
  delegation. Verified end-to-end against live GPT-5.4.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker/.env.example

@@ -333,16 +333,6 @@ STT_SERVICE=local/base
 # GATEWAY_DISCORD_ENABLED=FALSE
 # GATEWAY_DISCORD_REDIRECT_URI=http://localhost:3929/api/v1/gateway/discord/callback
 
-# ------------------------------------------------------------------------------
-# SearXNG (bundled web search, works out of the box with no config needed)
-# ------------------------------------------------------------------------------
-# SearXNG provides web search to all search spaces automatically.
-# To access the SearXNG UI directly in dev/deps-only compose: http://localhost:8888
-# To disable the service entirely: docker compose up --scale searxng=0
-# To point at your own SearXNG instance instead of the bundled one:
-# SEARXNG_DEFAULT_HOST=http://your-searxng:8080
-# SEARXNG_SECRET=surfsense-searxng-secret
-
 # ------------------------------------------------------------------------------
 # Daytona Sandbox (optional cloud code execution for the deep agent)
 # ------------------------------------------------------------------------------
@@ -492,9 +482,6 @@ NOLOGIN_MODE_ENABLED=FALSE
 # -- Redis exposed port (dev/deps-only only; Redis is internal-only in prod) --
 # REDIS_PORT=6379
 
-# -- SearXNG exposed port (dev/deps-only only; internal-only in prod) --
-# SEARXNG_PORT=8888
-
 # -- WhatsApp bridge exposed port (dev/hybrid only; prod keeps it Docker-internal) --
 # WHATSAPP_BRIDGE_PORT=9929