refactor: define SDK conversation host boundary

[roackb2]

Stack

• base: feat: add headless conversation agent #256 (codex/headless-conversation-agent)
• this PR contains one commit: 8f7fc3c5 refactor: organize SDK conversation hosts
• merge feat: add headless conversation agent #256 first, then retarget this PR to main

Summary

• keep runQuickstartConversationCli as the tiny console evaluation and integration host
• move the headless agent and quickstart console out of src/core/chat/engine into src/sdk/conversation/{headless,console}
• extract src/sdk/conversation/runtime as their single owner for starter defaults and credential preflight
• preserve all existing root-package API names and usage
• keep persisted session, turn, compaction, and persistence meaning in the core engine
• add transparent headless, hosted, and HTTP/SSE starter recipes with a lightweight readiness checklist
• document why SDK conversation hosts differ from the Heddle CLI/TUI, server, and web products
• add only two narrow dependency-direction assertions; no new conformance framework

Testing surface decision

This final diff does not introduce @roackb2/heddle/testing, src/testing, or public in-memory persistence adapters.

Persistence-only test adapters did not provide a complete deterministic agent-testing story, while the default file backend already supports zero-database startup. A coherent test harness can be reconsidered after concrete adopter demand; this PR does not create a permanent public API for a theoretical convenience.

Ownership boundary

• src/core/chat/engine: persisted conversation semantics and reusable domain services
• src/sdk/conversation/runtime: shared SDK-host defaults and credential preflight
• src/sdk/conversation/headless: structured in-process application service
• src/sdk/conversation/console: temporary readline and text SDK host
• src/cli-v2, src/server, src/web-v2: Heddle product interfaces and workflows

Verification

• focused SDK and architecture tests: 3 files / 28 tests
• yarn test: 121 unit files / 717 tests; 34 integration files / 310 tests
• yarn typecheck
• yarn lint
• yarn build
• built root-package export smoke for headless and quickstart services
• built testing entrypoint absent and package export absent
• 159 relative Markdown links resolved