fix(installer): derive installer version from the git tag, not a hardcoded default

installer.iss had #define MyAppVersion "2.1.0", so every release's setup.exe
(and its AppVersion/VersionInfoVersion) self-identified as 2.1.0 regardless of
the actual tag. setup-build-env.ps1 now passes /DMyAppVersion derived from
GITHUB_REF_NAME (CI tag run) or git describe, falling back to a 0.0.0-dev
placeholder in the .iss only for bare manual compiles.

installer/windows/installer.iss

@@ -5,10 +5,14 @@
 ; runtime. Nothing on the target machine requires internet access.
 ;
 ; Usage:
-;   iscc installer\windows\installer.iss
-;   theta-agent-2.1.0-windows-amd64-setup.exe /SILENT ^
+;   iscc "/DMyAppVersion=2.2.0" installer\windows\installer.iss
+;   theta-agent-2.2.0-windows-amd64-setup.exe /SILENT ^
 ;       /SERVER_URL=https://sso.example.com /JOIN_KEY=tjk_...
 ;
+; The version is passed in by scripts/setup-build-env.ps1 (derived from the
+; git tag). The default below exists only so a bare `iscc installer.iss` call
+; still compiles; it should never be the version in a real build.
+;
 ; Interactively, a wizard page asks for the Theta Directory URL and a join key
 ; (with a button that opens the Theta Directory's Directory -> Install Agent page
 ; to mint one). In silent mode, /SERVER_URL, /JOIN_KEY, /AUTH_TOKEN, /PUBLIC_KEY
@@ -16,7 +20,7 @@
 ; are written into agent.yml so the installed service enrolls on first start.
 
 #ifndef MyAppVersion
-  #define MyAppVersion "2.1.0"
+  #define MyAppVersion "0.0.0-dev"
 #endif
 
 #define MyAppName "Theta Agent"