Add library exercises mid-workout with plan defaults

Pick any exercise from the full library while a workout is running, not
just the ones in its split. The new exercise's plan is seeded from the
most recent log of that exercise, else the library's authored Defaults
line, else a plain 3x10. Adds a searchable two-section picker sheet and
a **Defaults:** bullet to all 47 library reference pages.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Cat-Cow/info.md

@@ -6,6 +6,7 @@ Gentle spinal mobility drill that wakes up the whole trunk before core work.
 - **Type:** Spinal mobility
 - **Targets:** Thoracic and lumbar spine, deep core awareness
 - **Prescription:** 10 slow reps (one rep = cat + cow)
+- **Defaults:** 3 × 10 bodyweight
 
 ## Setup