Add /graph G2: resumable budget, GraphUpdated status chip, PTY + turn-level coverage

BudgetLimited is now a resumable state: a budget trip demotes in-flight
nodes to Ready (a resource stop, not a verdict — no forever-Running node is
ever persisted) and '/graph resume --budget <tokens>' re-arms the graph with
fresh headroom (new budget = spent-so-far + extra). The tripped node's
partial burn is charged into tokens_spent_nodes at BOTH cascade sites before
the demotion clears current_node, so the top-up arithmetic never runs on an
under-counted ledger. Any input starting with 'resume' resolves to a resume
(case-insensitive; malformed top-ups surface the usage hint) and setup_graph
refuses to replace any non-Complete graph — a typo can no longer silently
destroy a resumable graph. An explicit --budget on a merely-paused graph is
rejected loudly instead of silently discarded; all trip-time messages now
advertise the top-up.

The pager gains a graph status chip: a new GraphUpdated wire variant
(extensions/notification.rs, old pagers degrade via #[serde(other)]) is
emitted from the single persist_graph_state chokepoint — every transition is
both a checkpoint and a badge tick — with a 'cleared' sentinel on /graph
clear and a one-shot re-emit after session restore (the replayed updates log
otherwise shows the pre-shutdown Active state that from_snapshot just
demoted in memory). TUI side: GraphDisplayState, session-notification arm,
and a goal-idiom chip with node progress, clamped current-node title, and
budget-aware spend. Pre-session command availability now advertises /graph
from the flags (it was fail-closed to the in-session path only, so the
welcome-screen slash menu never showed it).

Coverage: GraphUpdated wire round-trip + minimal-payload + unknown-tag
tests; PTY scenarios graph_slash_presession{,_disabled}.yaml (both run
green against the real pager binary); handle_prompt-level e2e for terminal
slash outcomes (/graph status|resume|pause, /goal refusals while the graph
owns the engine); budget top-up e2e driving a BudgetLimited diamond back to
Complete. Not shimmed: pre-G2 persisted snapshots with budget-Failed nodes
(the KIGI_GRAPH flag has never shipped enabled, so none exist).

kigi-shell 4927 and kigi-tui 6610 lib tests green; workspace clippy clean.

crates/codegen/kigi-tui/tests/scenarios/graph_slash_presession.yaml

@@ -0,0 +1,40 @@
+name: graph-slash-presession
+description: >
+  With the graph feature flag on (`KIGI_GRAPH=1`, plus `KIGI_GOAL=1` — the
+  graph gate requires the goal harness), `/graph` must appear in the
+  slash-command menu on the welcome screen *before* the first user turn
+  creates a session. Typing `/graph` pre-session must surface the command
+  and its description in the dropdown.
+terminal:
+  rows: 40
+  cols: 100
+environment:
+  env:
+    - key: KIGI_GOAL
+      value: "1"
+    - key: KIGI_GRAPH
+      value: "1"
+mock:
+  response: "unused — this scenario never submits a prompt."
+steps:
+  - action: wait_for_text
+    text: Quit
+    timeout_ms: 20000
+  - action: assert_not_contains
+    text: panicked
+  # Sanity: nothing has been submitted yet, so no session exists.
+  - action: assert_not_contains
+    text: dependency graph
+  - action: focus_prompt
+  - action: type_text
+    text: "/graph"
+  - action: wait
+    millis: 300
+  # The dropdown row carries the builtin's description, which only renders
+  # when /graph is actually advertised pre-session.
+  - action: assert_contains
+    text: dependency graph
+  - action: assert_running
+  - action: screenshot
+    name: graph-in-slash-menu-presession
+    note: "/graph advertised in the slash menu on the welcome screen (KIGI_GRAPH=1), before any prompt."

crates/codegen/kigi-tui/tests/scenarios/graph_slash_presession_disabled.yaml

@@ -0,0 +1,45 @@
+name: graph-slash-presession-disabled
+description: >
+  Fail-closed counterpart: with `KIGI_GRAPH=0` (goal harness still on),
+  `/graph` must NOT appear in the pre-session slash menu, while the menu
+  itself keeps working for other commands.
+terminal:
+  rows: 40
+  cols: 100
+environment:
+  env:
+    - key: KIGI_GOAL
+      value: "1"
+    - key: KIGI_GRAPH
+      value: "0"
+mock:
+  response: "unused — this scenario never submits a prompt."
+steps:
+  - action: wait_for_text
+    text: Quit
+    timeout_ms: 20000
+  - action: assert_not_contains
+    text: panicked
+  - action: focus_prompt
+  - action: type_text
+    text: "/graph"
+  - action: wait
+    millis: 300
+  - action: assert_not_contains
+    text: dependency graph
+  - action: assert_running
+  - action: screenshot
+    name: graph-absent-from-slash-menu
+    note: "/graph hidden with KIGI_GRAPH=0 (fail-closed gate)."
+  # The menu itself still works: clear "/graph" (6 chars) and try /compact.
+  - action: keys
+    keys: "<BS><BS><BS><BS><BS><BS>"
+  - action: type_text
+    text: "/compact"
+  - action: wait
+    millis: 300
+  - action: assert_contains
+    text: Compact conversation history
+  - action: screenshot
+    name: slash-menu-still-works
+    note: "Slash menu functional; only /graph is gated off."