fix(ssh): switch CI SSH to permissive mode (no known_hosts)

Root cause: ssh-keygen -R prints 'Host X not found in F:\CI\State\known_hosts'
to stderr when the IP has no prior entry. In PS5.1 with ErrorActionPreference=Stop,
native stderr is converted to ErrorRecords before 2>$null can discard them,
making the command a terminating error caught by the job's catch block.

This caused all 4 concurrent burn-in jobs to fail after ~15s with:
  Error: Host 192.168.79.XXX not found in F:\CI\State\known_hosts

Additionally, 4 concurrent SSH connections writing to the same known_hosts file
with no advisory locking risk file corruption under concurrent load.

Fix: set SshKnownHostsFile default to '' in Invoke-CIJob, Invoke-RemoteBuild,
and Get-BuildArtifacts. When KnownHostsFile='' _Transport.psm1 uses
StrictHostKeyChecking=no + UserKnownHostsFile=NUL (permissive mode), consistent
with Wait-VMReady and Prepare-LinuxBuild2404.ps1. Ephemeral CI VMs on a
local NAT network have no meaningful MITM risk.

Remove the ssh-keygen purge block from Invoke-CIJob.ps1 (no longer needed).

AGENTS.md: document PS5.1 native stderr / 2>$null interaction (error #12).

scripts/Invoke-CIJob.ps1

@@ -147,8 +147,10 @@ param(
     [string] $SshUser    = 'ci_build',
 
     # Persistent known_hosts file used for CI-job SSH calls.
-    # Set to empty string to disable host-key checking (template provisioning only).
-    [string] $SshKnownHostsFile = 'F:\CI\State\known_hosts',
+    # Empty string (default) = permissive mode (StrictHostKeyChecking=no, UserKnownHostsFile=NUL).
+    # Ephemeral CI VMs on a local NAT network do not need strict host-key verification.
+    # Pass a file path only if you need accept-new mode for a specific deployment.
+    [string] $SshKnownHostsFile = '',
 
     # Extra environment variables injected into the guest VM before the build command (§6.5).
     # Keys must be valid environment variable names. Values must be plain strings.
@@ -438,20 +440,6 @@ try {
             Write-Host "[Invoke-CIJob] Could not detect GuestOS from VMX — defaulting to Windows"
         }
     }
-    # ── Purge stale known_hosts entry for this IP (Linux only) ───────────
-    # Each clone gets a new SSH host key. Remove any previous entry for this
-    # IP from the CI-local known_hosts file before the first SSH connection,
-    # so ssh.exe uses accept-new instead of refusing a changed key.
-    if ($GuestOS -eq 'Linux' -and $SshKnownHostsFile -ne '') {
-        $khDir = Split-Path $SshKnownHostsFile -Parent
-        if (-not (Test-Path $khDir)) {
-            New-Item -ItemType Directory -Path $khDir -Force | Out-Null
-        }
-        if (Test-Path $SshKnownHostsFile) {
-            & ssh-keygen -R $VMIPAddress -f $SshKnownHostsFile 2>$null | Out-Null
-            Write-Host "[Invoke-CIJob] Purged known_hosts entry for $VMIPAddress (fresh clone)."
-        }
-    }
     # ── Phase 4: Wait for readiness ───────────────────────────────────────
     Write-Host "`n[Phase 4/6] Waiting for VM readiness..."
     Write-JobEvent -Phase 'phase4.wait-ready' -Status 'start' -Data @{ ip = $VMIPAddress }