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.sh

@@ -209,7 +209,6 @@ case "${SHELL:-}" in
     */zsh)
         RC_FILE="${ZDOTDIR:-$HOME}/.zshrc"
         PATH_LINE="export PATH=\"$BIN_DIR:\$PATH\""
-        GRAPH_LINE="export KIGI_GRAPH=1"
         ;;
     */bash)
         # macOS login shells read ~/.bash_profile; Linux reads ~/.bashrc.
@@ -219,7 +218,6 @@ case "${SHELL:-}" in
             RC_FILE="$HOME/.bashrc"
         fi
         PATH_LINE="export PATH=\"$BIN_DIR:\$PATH\""
-        GRAPH_LINE="export KIGI_GRAPH=1"
         ;;
     */fish)
         # fish_add_path in config.fish is fish's own idempotent way
@@ -228,12 +226,10 @@ case "${SHELL:-}" in
         mkdir -p "$FISH_CONF_DIR"
         RC_FILE="$FISH_CONF_DIR/config.fish"
         PATH_LINE="fish_add_path $BIN_DIR"
-        GRAPH_LINE="set -gx KIGI_GRAPH 1"
         ;;
     *)
         RC_FILE="$HOME/.profile"
         PATH_LINE="export PATH=\"$BIN_DIR:\$PATH\""
-        GRAPH_LINE="export KIGI_GRAPH=1"
         ;;
 esac
 
@@ -247,8 +243,8 @@ case ":$PATH:" in
         ;;
 esac
 
-# Graph engineering ships enabled by default. The KIGI_GRAPH guard makes
-# this idempotent AND respects an explicit user opt-out (an existing
-# `export KIGI_GRAPH=0` line is left untouched). Disable any time with:
+# Graph engineering is enabled by default IN THE BINARY (resolve_graph
+# defaults true) — the installer no longer writes KIGI_GRAPH=1 into shell
+# rc files (per-shell env plumbing was fragile and diverged per platform).
+# Disable any time with:
 #   echo 'export KIGI_GRAPH=0' >> <your shell rc>
-persist_line "$RC_FILE" "$GRAPH_LINE" "KIGI_GRAPH" "graph engineering (KIGI_GRAPH=1)"