docs: bring README, skill and dist up to date through P5a

README gains API, CLI and configuration sections and the PDF range/grep/question limits; the skill covers PDF busy and failure results; dist/sift.service matches the Nix unit (MemoryMax=1G, secret.env); the example config drops the unused [upstreams] block; stale task comments in api/pdf.go.

Gate overridden by the owner: TestGrepLimits (given test) exceeds its 1 s budget under -race on this host.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

dist/sift.toml.example

@@ -1,5 +1,6 @@
-# sift configuration. P0 uses -listen and -deny-prefix; remaining keys are P1+.
-# This file is an example: the service does not assume Nix store paths.
+# sift configuration (/etc/sift/config.toml, or $SIFT_CONFIG). Every key is optional;
+# commented values are examples. The service does not assume Nix store paths.
+# Paid-search keys (BRAVE_API_KEY, TAVILY_API_KEY) go in /var/lib/sift/secret.env, not here.
 
 listen = "127.0.0.1:8890"
 
@@ -8,11 +9,6 @@ listen = "127.0.0.1:8890"
 # Set on-box; do not commit residential addresses to git.
 # deny_prefixes = ["203.0.113.0/24"]
 
-# [upstreams]
-# searxng = "http://127.0.0.1:8888"
-# dixie   = "http://dixie.scylla-hammerhead.ts.net:11434/v1"
-# honcho  = "http://rift.scylla-hammerhead.ts.net:8000"
-
 # user_agent = "sift/dev"
 
 # [self_ip]