install: stop persisting KIGI_GRAPH — the binary default is the product default

The README always shipped graph engineering enabled; the enablement was
delegated to installer env plumbing that diverged per platform:
install.sh exported KIGI_GRAPH=1 into shell rc (worked), install.ps1
wrote the User registry variable — which running Windows terminals (and
new tabs of an open Windows Terminal) never pick up, so /graph was
'missing on Windows' despite a successful install.

With resolve_graph() defaulting true in the binary (e53a66d), the env
writes are redundant complexity: drop them from both installers, keep
KIGI_GRAPH=0 as the documented opt-out (env still beats the default),
and correct the flag comment to tell this story instead of a
'gray release' one. Both scripts syntax-checked (sh -n / pwsh parser).

Installers are served from main (raw.githubusercontent), so this takes
effect for all new installs immediately — no retag needed; the running
v0.1.5 build already carries the binary-default fix.

install.ps1

@@ -149,15 +149,13 @@ try {
         Write-Host "Run 'kigi' to get started."
     }
 
-    # Graph engineering ships enabled by default. Respect an explicit
-    # user choice: only set the variable when it is not already defined
-    # (so a persisted opt-out of "0" survives reinstalls).
-    $Graph = [Environment]::GetEnvironmentVariable("KIGI_GRAPH", "User")
-    if ($null -eq $Graph -or $Graph -eq "") {
-        [Environment]::SetEnvironmentVariable("KIGI_GRAPH", "1", "User")
-        Write-Host "Enabled graph engineering (KIGI_GRAPH=1)."
-        Write-Host "Disable: [Environment]::SetEnvironmentVariable('KIGI_GRAPH','0','User')"
-    }
+    # Graph engineering is enabled by default IN THE BINARY (resolve_graph
+    # defaults true) — no environment plumbing needed. The installer used
+    # to persist KIGI_GRAPH=1 into the User registry env, but running
+    # terminals (and new tabs of an open Windows Terminal) never pick up
+    # freshly-written registry variables, which made /graph "missing on
+    # Windows" while the shell-rc path worked on macOS/Linux. Opt out any
+    # time with: [Environment]::SetEnvironmentVariable('KIGI_GRAPH','0','User')
 } finally {
     Remove-Item -Path $TmpDir -Recurse -Force -ErrorAction SilentlyContinue
 }