feat: bake version into binary and display in UI sidebar

- Inject VERSION via ldflags (-X main=web.Version=) and git short SHA
- Add /api/version endpoint returning {version, commit}
- Display version in Layout sidebar footer (v0.1.6 · 8cfd405)
- Add AGENTS.md section documenting SemVer policy and bump-before-push rule
- Bump VERSION 0.1.5 -> 0.1.6 in Makefile

AGENTS.md

@@ -52,3 +52,24 @@ cd web && npm run dev   # Vite on :5173, proxies /api to :8080
 ```
 
 Requires `web/node_modules` (run `cd web && npm install` first).
+
+## Releasing / Versioning
+
+The project follows **SemVer**: `MAJOR.MINOR.PATCH`.
+
+- **PATCH** (`0.1.5 → 0.1.6`): bug fixes, no API changes.
+- **MINOR** (`0.1.x → 0.2.0`): new features, backward-compatible.
+- **MAJOR** (`0.x → 1.0`): breaking changes.
+
+**Bump before commit/push.** Update `VERSION?=` in the `Makefile` before staging any
+release-worthy changes. The value is baked into the binary at build time via
+`-ldflags -X github.com/darroyo/nasctl/internal/web.Version=$(VERSION)` and surfaced
+in the UI sidebar footer and `GET /api/version`.
+
+The short git SHA is also injected automatically via
+`-X github.com/darroyo/nasctl/internal/web.Commit=$(shell git rev-parse --short HEAD)`.
+
+Override at build time: `make build VERSION=0.2.0-rc1`.
+
+When reviewing staged changes, always confirm the version bump matches the
+nature of the change (patch vs minor vs major).