docs: update documentation for Linux VM support (Sprint 9)

AGENTS.md (new):
- Environment reference for AI agents: host specs, PS 5.1 constraints,
  dir structure, VM templates, transport layer, style rules
- Lessons #9 (snapshot cold state) and #10 (vmrun list vs getGuestIPAddress)

ARCHITECTURE.md:
- Add SSH/SCP transport section (Linux VMs)
- Update system diagram to include _Transport.psm1, SSH transport path
- Network layout: add SSH port 22 for Linux VMs
- Timeline: Linux readiness check phases documented

CI-FLOW.md:
- Prerequisites table: add Template VM (Linux), SSH key row
- Step 6 (readiness): document Windows vs Linux phases
- Step 7 (build) and Step 8 (artifacts): document Linux/SSH paths
- ci-report-ip guestinfo mechanism referenced

HOST-SETUP.md:
- Directory tree: add LinuxBuild2404 template dir and keys/
- Post-setup checklist: add step 5 (SSH key generation + Linux template)
- Fix duplicate step numbering (56, 67)
- Reference Paths table: add Linux template VMX, snapshot, SSH key

LINUX-TEMPLATE-SETUP.md:
- Rewritten as implementation record (no longer a plan)
- Fase A user-data: minimal (user+SSH key only, no packages/runcmd)
- All phases marked complete with actual paths and snapshot names

WINDOWS-TEMPLATE-SETUP.md: minor updates for consistency

TODO.md:
- Summary: update last-updated to 2026-05-11, Sprint 9 description
- Linux Build VM row: all 5 items complete
- Prossimi passi: Test Plan + §6.6 Tier-2 as next priorities

docs/HOST-SETUP.md

@@ -33,7 +33,11 @@ F:\CI\
 ├── Artifacts\              # output build raccolti dall'host
 ├── Logs\                   # log per-job (retention: 30 giorni)
 ├── Templates\
-│   └── WinBuild2025\       # VMX template Windows (immutabile dopo snapshot)
+│   ├── WinBuild2025\       # VMX template Windows (immutabile dopo snapshot)
+│   └── LinuxBuild2404\     # VMX template Linux Ubuntu 24.04 (immutabile dopo snapshot)
+├── keys\
+│   ├── ci_linux            # chiave privata SSH per Linux VMs (ed25519, no passphrase)
+│   └── ci_linux.pub        # chiave pubblica corrispondente
 ├── act_runner\
 │   ├── act_runner.exe      # binario runner Gitea
 │   ├── config.yaml         # config runner (path, label, capacity)
@@ -125,13 +129,21 @@ Allo `Setup-Host.ps1` exit 0:
    ```
    *Note*: Prepare-WinBuild2025.ps1 aggiunge l'IP specifico della VM in modo transitorio e lo ripristina nel `finally`.
 
-4. **Provisiona il template VM** — vedi [WINDOWS-TEMPLATE-SETUP.md](WINDOWS-TEMPLATE-SETUP.md)
+4. **Provisiona il template VM Windows** — vedi [WINDOWS-TEMPLATE-SETUP.md](WINDOWS-TEMPLATE-SETUP.md)
 
-5. **Registra chiave SSH con Gitea**:
+5. **(Opzionale) Genera chiave SSH CI per Linux VMs** — necessaria solo se si vuole usare `linux-build`:
+   ```powershell
+   New-Item -ItemType Directory -Force F:\CI\keys | Out-Null
+   ssh-keygen -t ed25519 -f F:\CI\keys\ci_linux -N '""' -C 'ci-linux-runner'
+   # Risultato: F:\CI\keys\ci_linux (privata) + F:\CI\keys\ci_linux.pub (pubblica)
+   ```
+   Poi **provisiona il template VM Linux** — vedi [LINUX-TEMPLATE-SETUP.md](LINUX-TEMPLATE-SETUP.md)
+
+6. **Registra chiave SSH con Gitea**:
    - Gitea UI → User Settings → SSH Keys → Add Key
    - Chiave pubblica: `%USERPROFILE%\.ssh\id_rsa.pub`
 
-6. **Verifica runner online** in Gitea:
+7. **Verifica runner online** in Gitea:
    - `<GiteaUrl>/-/admin/runners`
    - Status atteso: **Online**, label `windows-build:host,dotnet:host,msbuild:host`
 
@@ -145,8 +157,11 @@ Allo `Setup-Host.ps1` exit 0:
 | act_runner exe            | `F:\CI\act_runner\act_runner.exe` (servizio: `act_runner`)   |
 | act_runner config         | `F:\CI\act_runner\config.yaml`                               |
 | act_runner logs           | `F:\CI\act_runner\logs\` (stdout.log, stderr.log)            |
-| Template VMX              | `F:\CI\Templates\WinBuild2025\WinBuild2025.vmx`              |
-| Snapshot name             | `BaseClean`                                                  |
+| Template VMX (Windows)    | `F:\CI\Templates\WinBuild2025\WinBuild2025.vmx`              |
+| Template VMX (Linux)      | `F:\CI\Templates\LinuxBuild2404\LinuxBuild2404.vmx`           |
+| Snapshot name (Windows)   | `BaseClean`                                                  |
+| Snapshot name (Linux)     | `BaseClean-Linux`                                            |
+| SSH key (Linux VMs)       | `F:\CI\keys\ci_linux` (privata), `F:\CI\keys\ci_linux.pub`   |
 | Clone base dir            | `F:\CI\BuildVMs\`                                            |
 | Artifact dir              | `F:\CI\Artifacts\`                                           |
 | Log dir                   | `F:\CI\Logs\` (retention: 30 giorni)                         |