Append pending sub-agent nudges onto existing ephemeral turns

[TheGreatAxios]

Summary

• Sub-agent withEphemeralNudge appends the pending nudge turn onto options.ephemeralTurns instead of replacing the array, matching ChatDirector's behavior — pre-seeded ephemeral turns now survive applyPendingNudge
• Adds a regression test covering an infer that already carries ephemeral turns when the tool-failure recovery nudge is applied

Verification

• bun run check: lint, typecheck, build pass; test suite 6706 pass / 4 fail — the 4 failures (createOptimizedContextStore, assembleChatAgent) fail identically on clean main and are unrelated to this change
• The new regression test fails before the fix (seeded turn dropped, ephemeralTurns length 1) and passes after (length 2, seed first, nudge appended)

Fixes CL-6719

[linear-code]

CL-6719