fix(desktop): align fresh-install home so upgraders don't brick

Two related first-launch bugs on machines with a legacy ~/.hermes:

- install.ps1 hardcoded $HermesHome/$InstallDir to %LOCALAPPDATA%\hermes
  and ignored the HERMES_HOME the desktop passes through. The desktop
  freezes HERMES_HOME at module load and prefers a legacy ~/.hermes when
  %LOCALAPPDATA%\hermes is absent, so the installer wrote to a different
  home than the shell read -> "Could not connect to Hermes gateway". Honor
  $env:HERMES_HOME in the param defaults.

- isBootstrapComplete() trusted the marker + checkout without verifying a
  runnable venv, so an interrupted/split install spawned a dead backend
  instead of re-bootstrapping. Also require the venv python to exist.

scripts/install.ps1

@@ -23,8 +23,8 @@ param(
     # exact ref.  Precedence: Commit > Tag > Branch.
     [string]$Commit = "",
     [string]$Tag = "",
-    [string]$HermesHome = "$env:LOCALAPPDATA\hermes",
-    [string]$InstallDir = "$env:LOCALAPPDATA\hermes\hermes-agent",
+    [string]$HermesHome = $(if ($env:HERMES_HOME) { $env:HERMES_HOME } else { "$env:LOCALAPPDATA\hermes" }),
+    [string]$InstallDir = $(if ($env:HERMES_HOME) { "$env:HERMES_HOME\hermes-agent" } else { "$env:LOCALAPPDATA\hermes\hermes-agent" }),
 
     # --- Stage protocol (additive; default invocation behaves as before) ----
     # See the "Stage protocol" section near the bottom of the file for the