feat(handoff): claim_pings/review_pings accept a list — ping every reviewer

Multi-reviewer setups (e.g. a correctness + a readability adversary) can now have
the watchdog ping ALL reviewers on a claim, each in its own session with its own
submit key. A bare string still works (single agent). _ping_agents() helper.

README.md

@@ -267,6 +267,9 @@ phases = [
   subject matches `claim_pattern` / `review_pattern`, and watches the two `inboxes` files. When a
   claim lands it pings the `claim_pings` agent; a review pings `review_pings`; an inbox change
   pings the relevant side. This is how the Builder and Adversary coordinate purely through git.
+  `claim_pings` / `review_pings` may be a single agent name **or a list** — e.g.
+  `claim_pings = ["correctness-adversary", "readability-adversary"]` pings every reviewer on a claim
+  (each in its own session), for multi-reviewer setups.
 
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