feat(chain): fail over on empty/degenerate responses

A failover chain previously treated a successful-but-empty completion (no
content parts and no tool calls — a "stop with nothing") as a valid result
and returned it. The agent loop then ended the run with empty output, and
the configured backup models were never tried because no error was raised.
This let a single flaky model silently terminate an agent/skill run with
no answer (observed in the wild with ollama-cloud/glm-5.2 returning empty
completions right after a large tool/think turn).

- Add llm.ErrEmptyResponse (classified transient) and Response.IsEmpty():
  true only when there are no tool calls and no meaningful content (no
  parts, or whitespace-only text). A media/image part counts as content,
  so image-only responses are NOT empty.
- chain.Generate converts an empty completion into ErrEmptyResponse so the
  chain fails over to the next target. Unlike an ordinary transient it is
  NOT retried on the same target (the model just produced it; these calls
  are expensive) — the chain penalizes health (so a persistently-empty
  target benches) and advances immediately.
- When every target returns empty the call fails with ErrChainExhausted
  joined to ErrEmptyResponse — a visible error instead of a hollow success.
  Single-element chains therefore also surface empties as errors.

Stream path is unchanged (can't inspect content before the consumer reads
it). Tests: Response.IsEmpty table; chain fails over past an empty head;
all-empty chain returns ErrChainExhausted/ErrEmptyResponse; repeated
empties bench the target across requests. Full suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -90,6 +90,15 @@ process env eagerly; unknown names also resolve lazily at Parse time
   benched for base 5s × 2^n, capped 5m. Success fully resets. Chains skip
   benched targets; 404 advances penalty-free; auth/malformed fail fast
   (configurable); exhaustion returns a joined error naming every target.
+- **Empty response = failover.** A target that returns *without error* but
+  with no usable output — no content parts and no tool calls (`Response.IsEmpty`;
+  a media/image part counts as content) — is treated as a per-target failure
+  (`llm.ErrEmptyResponse`, classified transient). Unlike an ordinary
+  transient it is **not** retried on the same target (the model just did
+  this; the call is expensive): the chain penalizes health and advances
+  immediately. If every target comes back empty the call fails with
+  `ErrChainExhausted` rather than a hollow "successful" empty completion, so
+  a single flaky model can't silently end an agent run with nothing.
 - Tracker is in-memory, process-local, clock-injected. No persistence.
 
 ## House conventions (mirror foreman)