feat(jobs): node-local job ownership and capability-gated job__* visibility

Graph LLM nodes now own the jobs they start, on every exit path. A new
node_job_scope on RequestContext records job ids started while a node
runs: the turn-end guardrail nags only about the node's own jobs
(parallel branches no longer see each other's), and the node executor
reaps — cancels and deregisters — anything left registered when the
node exits, including error, timeout, and retry-exhaustion paths.
Cross-node job handoff is no longer possible; a crashed node takes its
in-flight jobs with it.

With inheritance gone, job__* declarations are gated on capability:
the family is only declared when at least one declared tool would pass
job__start's whitelist (shared predicate: is_backgroundable_tool). One
carve-out — while a context still owns registered jobs (job started,
tool disabled mid-session), the lifecycle verbs stay declared so a
running job can never become unreachable; job__start alone disappears.
A graph node with tools: [] now sees no job__* tools at all.

Prompt instructions, tool declarations, and graph.example.yaml updated
to the node-local semantics; +7 tests, 8 visibility pins rewritten.

graph.example.yaml

@@ -37,10 +37,12 @@ reasoning_effort: null             # Default reasoning effort for `llm` nodes th
                                    # Only valid when the model declares reasoning_levels.
 
 max_concurrent_jobs: 5             # Max background jobs (`job__*` tools) running at once across the
-                                   # whole graph run. Jobs live in the run-wide supervisor and outlive
-                                   # the `llm` node that started them, so the budget is graph-wide —
-                                   # there is no per-node override. Overrides the global setting;
-                                   # 0 disables background jobs for this graph agent.
+                                   # whole graph run: every `llm` node (including parallel branches)
+                                   # draws from this one pool, so the budget is graph-wide — there is
+                                   # no per-node override. Jobs themselves are node-local: the node
+                                   # that starts a job must collect or cancel it before it ends, and
+                                   # anything left running at node exit is cancelled. Overrides the
+                                   # global setting; 0 disables background jobs for this graph agent.
 
 global_tools:                      # Tool universe an `llm` node's `tools:` whitelist draws from
   - web_search_coyote.sh