feat: add /checkpoint command for creating milestone tags

.claude/commands/checkpoint.md

@@ -0,0 +1,36 @@
+---
+description: Create checkpoint tag before major change or after important milestone
+---
+
+Create a checkpoint tag to mark a stable point before implementing major changes or after completing important work.
+
+**Tag format**: `CHECKPOINT_<NAME>_<short_version>`
+- Name should be uppercase with underscores (e.g., `BEFORE_ACTIVITY_TRACKER`, `AFTER_SESSION_EXTENSION`)
+- Short version is major.minor.patch extracted from project.env
+
+**Step 1: Get current version**
+Use the Bash tool to extract the short version:
+```bash
+grep "^VERSION=" project.env | sed 's/VERSION="//;s/_.*//'
+```
+
+**Step 2: Determine checkpoint name**
+Based on context, suggest a checkpoint name:
+- Before implementation: `BEFORE_<FEATURE_NAME>`
+- After completion: `AFTER_<FEATURE_NAME>`
+- Stable release point: `STABLE_<DESCRIPTION>`
+
+If unclear, ask the user for the checkpoint name.
+
+**Step 3: Create and push tag**
+Use the Bash tool to create annotated tag and push:
+```bash
+git tag -a CHECKPOINT_<NAME>_<version> -m "Checkpoint: <brief description>" && \
+git push origin CHECKPOINT_<NAME>_<version>
+```
+
+**Step 4: Confirm**
+Inform the user:
+- The full tag name created
+- That it was pushed to origin
+- How to revert to this checkpoint if needed: `git checkout CHECKPOINT_<NAME>_<version>`