refactor: extract stream parsing into its own protocol module

[TheGreatAxios]

Summary

• Moves the event schemas, SSE streaming parse, non-streaming JSON parse, block indexing and terminal-event detection from src/responses.ts into src/protocol/iterator.ts.
• src/responses.ts now holds the quirks resolver, header extractors and the factory that wires body and iterator together.
• tagSignature stays in body.ts next to its untag/replay counterpart; iterator.ts imports it.
• src/index.ts imports isResponsesStreamTerminal from its new location; the export list is unchanged.
• Updates the AGENTS.md layout section. Code is moved verbatim; no behavior change.
• Stacked on refactor: extract request body building into its own protocol module #13.

Verification

• bun run check (typecheck, lint, format:check, test): 39 pass, 2 skip, 0 fail
• bun run build emits dist/protocol/iterator.js

Closes CL-9317

[TheGreatAxios]

Reviewed the full diff against #13: event schemas, stream/JSON parsing and the block indexer move verbatim; responses.ts keeps only quirks, header extractors and wiring. Public exports from src/index.ts are unchanged. Check and build pass locally.

[TheGreatAxios]

Independent review findings addressed and force-pushed: commit message reworded. check passes.