feat(training-agent): JSON-only Accept + idempotency storyboard fixes (group C start)

[bokelley]

Summary

Starts Group C (individual specialisms). Closes 4 more storyboard steps; idempotency goes from 3 failures → 1.

• Accept header negotiation: MCP Streamable HTTP transport rejects JSON-only Accept with 406. SDK storyboard rawMcpProbe (used for `omit_idempotency_key` vectors) sends JSON-only and hit that wall. Rewrite the header (including Node's `rawHeaders` array — which `@hono/node-server` actually reads) to include SSE when client sent JSON-only, enable `enableJsonResponse` on the transport for single-shot JSON responses. No hot-path regression (buyer agents using @adcp/client already send both).
• Emit `replayed: false` on fresh `create_media_buy` responses: storyboard uses `field_value allowed_values: [false]` which fails on omission despite the spec's MAY-omit clause. Scoped to `create_media_buy` only — other mutating tools continue MAY-omit because several SDK response schemas aren't `.passthrough()` and reject the extra key. Tracked in adcp-client#774.
• Strip `recovery` from `IDEMPOTENCY_CONFLICT`: SDK's new cross-step assertion `idempotency.conflict_no_payload_leak` enforces a narrow envelope allowlist (code, message, status, retry_after, correlation_id, request_id, operation_id). Our `recovery: correctable` hint fell outside. Drop it on this specific error to satisfy the stolen-key read-oracle defense; other errors keep recovery.

Impact

• 43/56 clean, 321 steps passing (up from 43/56, 317 after main merge — +4 steps)
• `idempotency` universal: 3 failures → 1 (webhook-dedup on replay still pending investigation)
• `property_lists`: unchanged (avoided regression by scoping replayed)

Test plan

• Typecheck clean
• 1796/1836 server unit tests passing (1 flaky C2PA test unrelated)
• Storyboards: 43/56 clean, above CI floor (36/295)
• Direct-POST probe confirms JSON-only Accept now returns 200 with Content-Type: application/json
• Storyboard idempotency confirmed +2 steps closing

Follow-ups

• adcp-client#774 (schema passthrough) — unblocks emitting `replayed: false` on all mutating tools
• Webhook dedup on idempotency replay (last remaining idempotency failure)

🤖 Generated with Claude Code