Preserve delegation work through any upstream outage, not just budget

An exhausted budget was a special case of something more general: the
work is not at fault. A network blip, a timeout, a 5xx from the proxy,
or a rate limit that did not clear all leave a delegation branch that
may be perfectly good, and discarding it would throw away work for a
reason that has nothing to do with the work.

UpstreamUnavailable now names that family, with BudgetExhausted a
member of it — distinguishable where the difference matters, since a
budget will not clear within the hour while a network blip will. A
delegation stopped by any of them is "interrupted": branch kept,
intent left pending, and A3's resume rule picks it up.

The benefit of the doubt stops at the server's own door. A 4xx that is
not a rate limit means the request itself was wrong, and preserving
branches on the strength of one would hide a real bug — so those still
fail and clean up.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nisse-dev/src/nisse_dev/delegation.py

@@ -36,7 +36,7 @@ from nisse_dev.canonical import (
     ToolResult,
 )
 from nisse_dev.git import GitError, GitRepo
-from nisse_dev.llm import BudgetExhausted, LLMClient, LLMError
+from nisse_dev.llm import LLMClient, LLMError, UpstreamUnavailable
 from nisse_dev.reconcile import Reconciler
 from nisse_dev.state import Intent, StateStore
 from nisse_dev.tools import EXECUTOR_TOOLS, MUTATING_TOOLS, Workspace
@@ -330,10 +330,10 @@ class Delegation:
                     model=self._model,
                     system=system,
                 )
-            except BudgetExhausted as exc:
+            except UpstreamUnavailable as exc:
                 return DelegationResult(
                     "interrupted",
-                    f"external models are unavailable: {exc}",
+                    f"stopped by something upstream, not by the work: {exc}",
                     branch=branch,
                     artifacts=sorted(touched),
                     tokens=spent,