docs: add follow mode highlight & command legend design

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-03-07-follow-highlight-design.md

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+# Follow Mode Highlight & Command Legend
+
+## Problem
+
+With many NPCs on screen, it's hard to identify which NPC you're following. Follow mode also lacks a command legend (camera mode has one).
+
+## Design
+
+### Highlight Effect
+
+- Phaser `postFX.addGlow(0xff0000, outlineSize)` on the followed NPC's sprite
+- Toggle on/off with `1` key (only in follow mode)
+- Default: OFF
+- When cycling NPCs (left/right), move glow from old sprite to new sprite (if enabled)
+- Clear glow when switching back to camera mode
+
+### Mode-Specific Key Bindings
+
+The `1` key already gates on camera mode (`if (this.mode !== 'camera') return`). Add a parallel follow-mode branch:
+
+- Camera mode `1`: Spawn NPC (existing)
+- Follow mode `1`: Toggle highlight
+
+### Follow Mode Command Legend
+
+Reuse existing `CommandPanel` class with a second instance:
+
+| Key | Label |
+|-----|-------|
+| ← → | Cycle NPC |
+| 1 | Highlight |
+| TAB | Camera Mode |
+
+- Title: "FOLLOW" (matching camera panel's "COMMANDS" style)
+- Show when entering follow mode, hide when leaving
+- Same position/styling as camera mode command panel
+
+### State
+
+- `highlightEnabled: boolean` on GameScene (default `false`)
+- `followCommandPanel: CommandPanel` on GameScene (separate instance)
+
+### Data Flow
+
+1. Enter follow mode → show follow command panel, hide camera command panel
+2. Press `1` in follow mode → toggle `highlightEnabled`, apply/remove glow on current sprite
+3. Cycle NPC → if highlight enabled, remove glow from old sprite, add to new sprite
+4. Exit follow mode → clear glow, reset `highlightEnabled`, hide follow command panel