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>

internal/api/pdf.go

@@ -18,9 +18,10 @@ import (
 )
 
 const (
-	// DefaultPDFTTL bounds a cached PDF's life. Task 07; declared now so the test compiles.
+	// DefaultPDFTTL is how long a cached PDF is served before it is revalidated.
 	DefaultPDFTTL = 24 * time.Hour
-	// DefaultPDFMaxQuestionChunks caps a digest question's length. Task 07; declared now.
+	// DefaultPDFMaxQuestionChunks is the longest document (in digest chunks) a question is
+	// answered over; a longer one is refused in favour of grep or pages.
 	DefaultPDFMaxQuestionChunks = 200
 )