feat: add JSON batch operation reference (COD-443)

[shivros]

Summary

Proves Hydra's existing explicit json parameter capability can represent a source-agnostic ingest batch through one declared operation contract.

• Adds a tested ingest_batch reference operation in examples/notes with replay key, batch hash, a non-empty JSON event array, and explicit repeatable --event CLI representation.
• Regenerates committed CLI, HTTP, and MCP artifacts from that one definition.
• Documents the consumer boundary: Iris owns event typing, batch hashing, idempotency, and transactional persistence.

Verification

• cargo build --all-targets
• cargo test --all-targets (51 tests)
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• cd examples/notes && cargo run -p hydra-codegen --bin hydra-codegen -- check

Closes COD-443.

[shivros]

🤖 Automated Review Panel

GPT-5.6 (openai-codex/gpt-5.6-terra)

APPROVE — the explicit generic batch contract generates aligned HTTP, CLI, and MCP surfaces. Fixture-backed tests cover HTTP/dispatch parity, CLI parse plus dispatch normalization, and MCP schema; reviewer additionally exercised real CLI and MCP batch calls successfully. No consumer-specific persistence/idempotency claims were introduced.

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Gemini 3 Flash (google/gemini-3-flash-preview)

APPROVE — independently confirmed explicit JSON schema and CLI representation, deterministic generated artifacts, safe CLI JSON-object normalization, and consumer-owned idempotency/transaction semantics. No substantive correctness or goal-alignment issues found.

[shivros]

Auto-Merge Gate — approval rationale

Confidence: 0.93

The PR fulfills COD-443's bounded Hydra objective: it demonstrates one explicit JSON batch contract projected through generated HTTP, CLI, and MCP surfaces, with a single consumer dispatch implementation. The diff keeps Iris-specific event models, hashing, idempotency, and persistence out of Hydra.

Evidence

• GitHub CI: Test and Creed context drift succeeded.
• Local verification: cargo test -p notes-example --test surfaces (14 passed) and hydra-codegen check passed from the PR head.
• The committed generated MCP schema aligns with operations.yaml; tests cover HTTP/dispatch equivalence and CLI/MCP batch behavior.
• Dual-model review panel approved; no unresolved blocking comments were found.

Scope limits
This approval covers only the generic Hydra reference contract. Iris remains responsible for its store/transaction semantics and adoption.