Add changeset for v0.21 release (Fission-AI#516)

Author: TabishB

.changeset/v0.21-release.md

@@ -0,0 +1,17 @@
+---
+"@fission-ai/openspec": minor
+---
+
+### New Features
+
+- **Feedback command** — Submit feedback directly from the CLI with `openspec feedback`, which creates GitHub Issues with automatic metadata inclusion and graceful fallback for manual submission
+- **Nix flake support** — Install and develop openspec using Nix with the new `flake.nix`, including automated flake maintenance and CI validation
+
+### Bug Fixes
+
+- **Explore mode guardrails** — Explore mode now explicitly prevents implementation, keeping the focus on thinking and discovery while still allowing artifact creation
+
+### Other
+
+- Improved change inference in `opsx apply` — automatically detects the target change from conversation context or prompts when ambiguous
+- Streamlined archive sync assessment with clearer delta spec location guidance