chore: initialise la structure du projet SentinelMesh

- Workspace Cargo avec backend, agent-scan-network, agent-metric
- Skeleton Rust pour les trois crates (Axum, Tokio, SQLx)
- Documentation : README, FEATURES, ROADMAP, ARCHITECTURE, API, INSTALL
- Exemples de widgets Glance (custom-api)
- Script d'installation agents (squelette Phase 5)
- Docker Compose + Dockerfile backend
- .gitignore et CLAUDE.md

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

Cargo.toml

@@ -0,0 +1,15 @@
+[workspace]
+resolver = "2"
+members = [
+    "backend",
+    "agents/agent-scan-network",
+    "agents/agent-metric",
+]
+
+[workspace.dependencies]
+tokio       = { version = "1", features = ["full"] }
+serde       = { version = "1", features = ["derive"] }
+serde_json  = "1"
+anyhow      = "1"
+tracing     = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }