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/openapi/routes/ocr.ts

@@ -29,9 +29,7 @@ const capabilityResponse = z
   .object({
     available: z.boolean(),
     mode: z.enum(["vision", "text"]).nullable(),
-    reason: z
-      .enum(["no-provider", "text-only-model", "enable-local-ocr"])
-      .nullable(),
+    reason: z.enum(["no-provider", "enable-local-ocr"]).nullable(),
     pdfSupported: z.boolean(),
   })
   .meta({