fix: stacks STACKS_DIR defaults to PWD instead of ~/stacks

Makes stacks work like arty — discovers stacks in the current directory.
Explicit STACKS_DIR env var still takes precedence.

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

README.md

@@ -120,7 +120,7 @@ arty docker:install         # Docker Engine with user group
 Scripts live in `~/bin/` and are tracked in this repo.
 
 - **`arty`** — repository orchestration, reads `arty.yml`
-- **`stacks`** — Docker Compose stack manager for `~/stacks`
+- **`stacks`** — Docker Compose stack manager (runs in current directory, or set `STACKS_DIR`)
 
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