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

web/src/api.ts

@@ -65,6 +65,11 @@ export interface SystemStatus {
   services: ServiceStatus[];
 }
 
+export interface VersionInfo {
+  version: string;
+  commit: string;
+}
+
 export class ApiError extends Error {
   status: number;
   constructor(status: number, message: string) {
@@ -104,6 +109,7 @@ export const api = {
   apply: () => request<{ results: { module: string; applied: boolean; error?: string }[] }>("POST", "/apply"),
   applyLog: () => request<{ entries: ApplyLogEntry[] }>("GET", "/apply/log"),
   systemStatus: () => request<SystemStatus>("GET", "/system/status"),
+  version: () => request<VersionInfo>("GET", "/version"),
 
   // samba
   listShares: () => request<{ shares: SambaShare[] | null }>("GET", "/samba/shares/"),