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.service

@@ -28,7 +28,7 @@ RestrictRealtime=true
 RestrictSUIDSGID=true
 LockPersonality=true
 MemoryDenyWriteExecute=true
-MemoryMax=512M
+MemoryMax=1G
 SystemCallArchitectures=native
 SystemCallFilter=@system-service
 SystemCallFilter=~@privileged
@@ -37,6 +37,7 @@ AmbientCapabilities=
 ReadWritePaths=/var/lib/sift
 ConfigurationDirectory=sift
 StateDirectory=sift
+EnvironmentFile=-/var/lib/sift/secret.env
 
 [Install]
 WantedBy=multi-user.target