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>

docs/CHEATSHEET.md

@@ -32,14 +32,27 @@ make down    # podman-compose down
 cp config/example.yaml config/s5p.yaml     # create live config (gitignored)
 ```
 
-## Proxy Source (config)
+## Proxy Pool (config)
 
 ```yaml
-proxy_source:
-  url: http://10.200.1.250:8081/proxies
-  proto: socks5    # optional filter
-  limit: 1000
-  refresh: 300     # seconds
+proxy_pool:
+  sources:
+    - url: http://10.200.1.250:8081/proxies
+      proto: socks5
+      limit: 1000
+    - file: /etc/s5p/proxies.txt
+  refresh: 300       # re-fetch interval
+  test_interval: 120 # health test cycle
+  max_fails: 3       # evict after N fails
+```
+
+## Proxy File Format
+
+```
+# one proxy URL per line
+socks5://1.2.3.4:1080
+socks5://user:pass@5.6.7.8:1080
+http://proxy.example.com:8080
 ```
 
 ## Proxy URLs