fix(broadcast): close cost-guard gate gap (#24 follow-up — broadcast surface)

[suzuke]

Summary

PR #57 (the #24 fix) gated sendToInstance and — transitively via wrapAsSend — delegate_task and request_information. But broadcast runs its own per-target send loop without funnelling through sendToInstance, so a limited instance could still be flooded with broadcast task / query / update messages. ts-reviewer flagged this on PR #57 as Out-of-scope Observation 1. This PR closes the gap so the #24 issue intent ("notify sender on cost-guard limit") covers all outbound-dispatch surfaces an agent can use.

Changes

• src/outbound-handlers.ts.broadcast (~10 LOC): inside the per-target loop, when ctx.costGuard?.isLimited(target) is true, append the target to a new costLimited array with the same warning string format used in feat(#24): cost-guard pre-check at outbound dispatch #57 (cost-guard: instance '...' has reached its daily cost limit ($X.XX). ...) and continue instead of dispatching. Other targets keep flowing normally — broadcast is never short-circuited as a whole.
• broadcast result gains cost_limited: [{ target, warning }] alongside sent_to, failed, count. Callers can see exactly which targets were skipped and why.

Out of scope (per ts-lead)

• Session-routing edge case (Out-of-scope Observation 2 from PR feat(#24): cost-guard pre-check at outbound dispatch #57): gate keys by targetName directly, not by the resolved host. Deferred per dispatch.
• request_kind: "report" bypass: not applicable here. BroadcastRequestKind (outbound-schemas.ts) already restricts to "query" | "task" | "update" — broadcasts never carry reports because they have no correlation_id to terminate.

Test plan

• npx tsc --noEmit — clean
• npx vitest run — 63 files / 527 tests pass (+3 over post-feat(#24): cost-guard pre-check at outbound dispatch #57 baseline; no regressions)
• tests/outbound-cost-guard-precheck.test.ts — three operator-mandated states for broadcast:
  • (a) mixed → limited targets warned + skipped (no IPC send), others delivered. Result cost_limited contains exactly the limited target with the standard warning string.
  • (b) all limited → zero IPC sends across all targets, cost_limited lists all, sent_to empty, count: 0.
  • (c) no targets limited → identical to pre-fix behaviour (cost_limited: [], sent_to matches all targets).

Risks considered

• Result shape change: adds new cost_limited field. Existing callers reading sent_to / failed / count see no change. New field default is [], never null. No migration needed.
• Concurrent cost-guard state changes during loop: same race surface as pre-existing pause flow — gate shrinks the window per target but doesn't eliminate it. Not introduced here.
• Performance: isLimited() is O(1) (Map lookup + arithmetic). Per-target overhead negligible.

Files

• src/outbound-handlers.ts — broadcast loop gate + result field
• tests/outbound-cost-guard-precheck.test.ts — 3 broadcast tests appended

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