feat(graph): /graph ships on by default — end the KIGI_GRAPH gray release

resolve_graph() read only the KIGI_GRAPH env var with default(false)
(plan.md G0 gate), so /graph existed solely on machines whose environment
exported the dev flag — which presented as '/graph is missing on
Windows'. There was never any platform-conditional code: the Mac worked
because the dev env var was set there.

Default is now true (matching /goal's shipped state); KIGI_GRAPH=0
remains the off-switch, and availability still requires the goal harness
(BuiltinGate::Graph). AGENTS.md updated; new test pins fresh-install-on
plus env-zero-off.

Verified: kigi-shell 5262 + kigi-tui 6874 tests green, clippy clean.

AGENTS.md

@@ -89,8 +89,9 @@ node as one ordinary goal — the agentic loop lives INSIDE the node; the
 edges stay deterministic Rust. The harness appends a terminal
 `gn-final` verification node depending on every planner node.
 
-- Feature flag `KIGI_GRAPH=1` (default off); availability additionally
-  requires the goal harness (`BuiltinGate::Graph`).
+- Enabled by default (`KIGI_GRAPH=0` is the off-switch; the G0 gray
+  release is over); availability additionally requires the goal harness
+  (`BuiltinGate::Graph`).
 - Key modules (kigi-shell): `session/graph_tracker.rs` (pure state
   machine; reuses `GoalStatus`/`GoalPhase`/`GoalPauseReason`),
   `session/graph_plan.rs` (planner-JSON contract + validation + fnv id