docs: describe the rfc1918 trusted_proxies default

Update README.md, config.example.yml and the TODO.md completed entry so
they state that an omitted trusted_proxies key defaults to the RFC 1918
private ranges, an explicitly empty list trusts no one, and an explicit
list replaces the default.

Model: opus-4-8

TODO.md

@@ -33,7 +33,9 @@ exhaustion
 - 2026-09-21 trusted-proxy client IP resolution (closes #94): a
   `trusted_proxies` config key taking a list of CIDRs, parsed by the same
   `net/netip` list parser as `blocked_networks` (an invalid entry aborts
-  startup naming the key and value; omitted or empty trusts no one); a new
+  startup naming the key and value; an omitted key defaults to the RFC 1918
+  private ranges, an explicitly empty list trusts no one, and an explicit
+  list replaces the default); a new
   `internal/clientip` package resolves the client address by honoring
   `X-Forwarded-For` only when the direct peer is a trusted proxy, walking
   the chain right-to-left to the rightmost non-proxy entry, so a client