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.

AGENTS.md

@@ -110,6 +110,14 @@ edges stay deterministic Rust. The harness appends a terminal
   serially on the full goal engine. Concurrency=1 is byte-identical to
   the serial G0 path. Ceiling: a worker round exceeding the foreground
   subagent await budget (600s) is cancelled and retried via resume.
+- G2: `BudgetLimited` is resumable — a budget trip demotes in-flight
+  nodes to `Ready` (resource stop, not a verdict) and
+  `/graph resume --budget <tokens>` re-arms with fresh headroom. The
+  pager shows a graph status chip driven by the `GraphUpdated` wire
+  variant (`extensions/notification.rs`), emitted from the single
+  `persist_graph_state` chokepoint (checkpoint ⇔ badge tick); old
+  pagers degrade via `#[serde(other)] Unknown`. PTY scenarios:
+  `graph_slash_presession{,_disabled}.yaml`.
 
 ## Milestones (PRD §8.3)