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.

crates/codegen/kigi-shell/src/agent/config.rs

@@ -1881,12 +1881,15 @@ impl Config {
             .default(true)
             .resolve()
     }
-    /// Graph mode (`/graph`) master switch. Default ON — the gray release
-    /// (plan.md G0, `KIGI_GRAPH=1` only) is over; every install gets the
-    /// same commands (its absence on non-dev machines read as a platform
-    /// bug). `KIGI_GRAPH=0` remains the off-switch. Graph mode additionally
-    /// requires the goal harness (nodes execute as goals), enforced at
-    /// availability time, not here.
+    /// Graph mode (`/graph`) master switch. Default ON in the binary: the
+    /// README ships graph engineering enabled for every install, but the
+    /// old `default(false)` delegated enablement to installer env plumbing
+    /// (`install.sh` shell-rc export vs `install.ps1` registry write) — and
+    /// Windows terminals don't pick up freshly-written registry env, so
+    /// `/graph` went "missing on Windows". The product default lives HERE,
+    /// not in installers. `KIGI_GRAPH=0` is the off-switch. Graph mode
+    /// additionally requires the goal harness (nodes execute as goals),
+    /// enforced at availability time, not here.
     pub(crate) fn resolve_graph(&self) -> Resolved<bool> {
         BoolFlag::env("KIGI_GRAPH").default(true).resolve()
     }