Enable graph engineering by default in installers; feature it in the README

README leads with the claim under the title — the world's first CLI with
built-in graph engineering — plus a dedicated section covering the /graph
command set, the closed loop it runs (plan → parallel worktree workers →
adversarial verify → 3-way merge-back → replan → optimize → terminal
whole-objective verification), the repo-following .kigi/graph.jsonl state,
copy-paste disable commands for macOS/Linux and Windows, and the tuning
knobs.

install.sh: the rc-file resolution and persist_line helper are hoisted out
of the not-on-PATH branch and generalized (guard + label), so PATH
persistence is unchanged while KIGI_GRAPH=1 is persisted unconditionally
and idempotently for zsh/bash/fish/POSIX profiles — an existing
KIGI_GRAPH line (e.g. a user's =0 opt-out) is left untouched on
reinstall. install.ps1 mirrors it with a User-scope environment variable,
also only when unset. No binary change — installers and README ship from
main, so this needs no rebuild or release.

install.ps1

@@ -148,6 +148,16 @@ try {
     } else {
         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')"
+    }
 } finally {
     Remove-Item -Path $TmpDir -Recurse -Force -ErrorAction SilentlyContinue
 }