feat: show prerequisite status for locked skills in /progress

Add locked skills section to progress command showing prerequisites
with mastery status and "ready to learn" indicators.

Closes DOJ-2441

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

Author: dbejarano820

commands/progress.md

@@ -51,9 +51,9 @@ Skills Seen But Not Quizzed 🆕 ([count])
 ─────────────────────────────
 [Concepts encountered but never quizzed]
 
-Skills Locked 🔒
+🔒 Locked Skills
 ────────────────
-[Next few skills to unlock, with what prerequisite they need]
+[Concepts whose prerequisites are not all mastered — see step 3b]
 
 Recent Achievements 🏆
 ──────────────────────
@@ -64,6 +64,37 @@ Recent Achievements 🏆
 Free & Open Source — github.com/dojocodinglabs/code-sensei
 ```
 
+3b. Build the **Locked Skills** section by reading `data/concept-tree.json`:
+
+   - Iterate over every concept in every category of `concept-tree.json`.
+   - A concept is **locked** if it has at least one entry in its `prerequisites` array AND not all of those prerequisites appear in the user's `concepts_mastered` list.
+   - A concept is **ready to learn** when ALL prerequisites are in `concepts_mastered` but the concept itself is not yet mastered (it may or may not be in `concepts_seen`).
+   - For each locked concept, check each prerequisite ID against `concepts_mastered`:
+     - Mastered prerequisite → mark with ✅
+     - Unmastered prerequisite → mark with ❌
+   - Sort the locked concepts so that **"Ready to learn"** concepts (all prerequisites ✅) appear first, followed by concepts that still have unmet prerequisites (at least one ❌), ordered by fewest unmet prerequisites first.
+   - Skip concepts whose prerequisites array is empty — those are not locked, they are always available.
+   - Skip concepts the user has already mastered.
+   - Format each locked concept as one line:
+
+   ```
+   🔒 [concept-id] — needs: [prereq-id] [✅ or ❌], [prereq-id] [✅ or ❌]
+   ```
+
+   Append ` (Ready to learn!)` at the end of the line when all prerequisites are ✅.
+
+   Example output:
+
+   ```
+   🔒 Locked Skills
+   ────────────────
+   🔒 state-management — needs: react-components ✅, objects ✅ (Ready to learn!)
+   🔒 middleware — needs: routes ✅, servers ❌
+   🔒 effects — needs: state ❌, async-await ❌
+   ```
+
+   If there are no locked concepts (the user has mastered or seen everything), omit this section entirely.
+
 4. If the user is new (no profile), show a welcome instead:
 
 ```