Add /graph G3: dynamic replan from DISCOVERED work items

Workers, verifiers, and serial node goals can now surface out-of-scope work
as line-anchored 'DISCOVERED: <text>' markers (fence-stripped and
placeholder-filtered — the templates' own examples are fenced so verbatim
echoes never parse; worker summaries embedded in verifier prompts get the
marker neutralized alongside NODE_RESULT/NODE_VERDICT). Discoveries queue on
the orchestration as persisted state and fold into the graph at dispatch
boundaries: a replanner subagent produces a strictly APPEND-ONLY appendix,
validated against the live graph (existing-id deps allowed; edges onto
gn-final rejected — they would cycle the moment the final-gating extension
lands; Blocks deps on Failed/Blocked nodes rejected as DeadDep so the
attempt-2 feedback loop repairs the artifact). Installing an appendix bumps
plan_version, freezes an immutable graph.baseline.v{N}.json next to the
prior versions, extends gn-final's gate (demoting a Ready final back to
Waiting), and recomputes readiness.

DiscoveredFrom edges are audit metadata, never scheduling gates: an origin
is always terminal at replan time, so gating on it is either a no-op or a
permanent wedge — and a failed node's discoveries are still real work.
Replanning is bounded by KIGI_GRAPH_REPLAN_CAP (default 3; 0 disables it
quietly): past the cap, after the final node has achieved, or on replan
failure, discoveries drain to history only — a working graph is never
paused for a failed enhancement pass, and it always converges. The budget
gate now precedes the replan boundary (a budget-dead graph keeps its
discoveries queued for a later --budget top-up instead of spending two
replanner runs first), and both planner runners delete stale artifacts
before spawning so a child that responds without writing can never get a
previous pass's file validated as its own output.

Tests: validate_replan unit coverage (existing-id resolution,
DiscoveredFrom dedup, collisions, dead deps vs dead origins, terminal-node
edges, combined-graph cycles), tracker appendix/regate/audit-edge tests,
and two e2e flows — discovery → replan → appended node runs to Achieved
with both baselines frozen, and cap-0 draining to history while the graph
still converges. kigi-shell 4935 lib tests green; workspace clippy clean.

crates/codegen/kigi-shell/src/session/storage/jsonl/tests.rs

@@ -2778,6 +2778,8 @@ async fn graph_mode_state_round_trips_and_tombstones() {
         tokens_spent_nodes: 42,
         history: vec![],
         pause_message: None,
+        pending_discoveries: vec![],
+        replan_runs: 0,
     };
     adapter.write_graph_mode_state(&info, Some(&state)).await.unwrap();
     let loaded = adapter.load_session_without_updates(&info).await.unwrap();