docs: add proxy pool documentation

Update all docs for managed proxy pool: README, USAGE, CHEATSHEET,
PROJECT, TASKS, and example config. Document multi-source config,
proxy file format, health testing, persistence, and legacy compat.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/example.yaml

@@ -16,11 +16,26 @@ chain:
   # - socks4://proxy:1080            # post-Tor SOCKS4/4a proxy
   # - http://user:pass@proxy:8080    # post-Tor HTTP CONNECT proxy
 
-# Dynamic proxy source -- appends a random proxy after the static chain.
-# Fetches from an HTTP API and caches the list.
+# Managed proxy pool -- fetches from multiple sources, health-tests,
+# and rotates alive proxies per-connection after the static chain.
+# proxy_pool:
+#   sources:
+#     - url: http://10.200.1.250:8081/proxies
+#       proto: socks5        # optional: filter by protocol
+#       country: US          # optional: filter by country
+#       limit: 1000          # optional: max proxies to fetch
+#     - file: /etc/s5p/proxies.txt  # text file, one proxy URL per line
+#   refresh: 300             # re-fetch sources interval (seconds)
+#   test_interval: 120       # health test cycle interval (seconds)
+#   test_url: http://httpbin.org/ip  # URL for health checks
+#   test_timeout: 15         # per-test timeout (seconds)
+#   test_concurrency: 5      # parallel health tests
+#   max_fails: 3             # consecutive fails before eviction
+#   state_file: ""           # empty = ~/.cache/s5p/pool.json
+
+# Legacy proxy source (still supported, auto-converts to proxy_pool):
 # proxy_source:
 #   url: http://10.200.1.250:8081/proxies
-#   proto: socks5       # optional: filter by protocol
-#   country: US          # optional: filter by country
-#   limit: 1000          # optional: max proxies to fetch
-#   refresh: 300         # cache refresh interval (seconds)
+#   proto: socks5
+#   limit: 1000
+#   refresh: 300