Add the hinge pattern: Deadlift, RDL, Glute Bridge, Hip Thrust, Back Extension

The biggest hole in the coverage matrix - the hinge had zero entries in
any modality. Deadlift and Romanian Deadlift are the library's first
standing (camera.zoom) and first barbell motions.

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

Exercise Library/Deadlift/info.md

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+# Deadlift
+
+The barbell hip hinge — brace hard, then stand up with the bar by driving
+the hips through.
+
+- **Category:** Main circuit
+- **Type:** Free-weight compound hinge
+- **Targets:** Glutes, hamstrings, spinal erectors, grip
+- **Prescription:** 4 × 6–8
+
+## Setup
+
+Stand with the bar over mid-foot, feet hip-width. Hinge down with a flat
+back and grip the bar just outside your legs. Shoulders slightly ahead of
+the bar, hips higher than knees, chest up.
+
+## Execution
+
+1. Take the slack out of the bar, brace your trunk, then push the floor
+   away — hips and shoulders rise together.
+2. Once the bar passes the knees, drive the hips forward to stand tall.
+3. Lower under control by hinging at the hips first, then bending the
+   knees once the bar clears them.
+
+## Cues
+
+- Flat back from setup to lockout — the spine never rounds or overarches.
+- Keep the bar dragging close to the legs the whole way.
+- Finish with glutes, not by leaning back.
+
+## Common Mistakes
+
+- Squatting the weight up — hips too low, knees pushed forward.
+- Rounding the lower back off the floor.
+- Jerking the bar off the ground instead of pushing through the legs.
+
+## Progression
+
+Add weight in small jumps while all reps stay flat-backed; start from
+blocks or a rack if floor mobility isn't there yet.