chore(labs): tidy OCR capability enum, Fitbit key delimiter, Coach docblock

Drop the unreachable text-only-model reason from the OCR capability
enum and its DTO type. Replace the literal NUL delimiter in the Fitbit
dedup map keys with a named constant so text tooling stops treating the
file as binary (behaviour unchanged). Correct the Coach tool-executor
docblock, which overstated cross-tool snapshot-cache sharing.

src/lib/validations/labs-ocr.ts

@@ -207,7 +207,7 @@ export interface OcrCapabilityDto {
    */
   mode: "vision" | "text" | null;
   /** Why scanning is unavailable, when it is. */
-  reason: "no-provider" | "text-only-model" | "enable-local-ocr" | null;
+  reason: "no-provider" | "enable-local-ocr" | null;
   /** Whether PDF uploads are accepted (Anthropic vision provider only). */
   pdfSupported: boolean;
 }