feat(packages): activate execution host client

[roackb2]

Summary

• move the canonical Execution Host adopter implementation into @heddleagent/execution-host-client
• activate only this v6 package as 6.0.0-next.0; keep the other four @heddleagent/* foundations private and unpublished
• preserve the TypeScript and clean-room Python conformance suites, including the durable hosted-conversation lifecycle profile from Add durable hosted conversation lifecycle #340
• add fail-closed package, pack, fresh-consumer, CI, and next-only release gates

Dependency state

#340 merged into main as ecf7ee2b. This branch is synchronized with that
merge and now targets main; the remaining diff is the package activation/move
only. The ancestry-only synchronization commit has the same tree as the
previously validated activation commit 4d00f2b8.

No npm publication is part of this PR. The first immutable next publication remains a separate, explicitly authorized release operation after merge.

Boundaries

• no runtime, CLI, run-client, or PostgreSQL package migration
• no duplicate old/new implementation tree
• no deprecation or unpublish of @roackb2/heddle-adopter@5.13.0
• no managed Execution Host or Python SDK claim
• no Lucid adapter or product-history implementation

Verification

• yarn package-family:verify
• yarn execution-host-client:contract:verify
• yarn execution-host-client:test (110 tests)
• Python v1 conformance: Ruff plus 35 tests on Python 3.12
• yarn typecheck
• yarn lint
• yarn test (876 unit, 428 integration, 110 package tests; environment-gated PostgreSQL cases skipped)
• yarn build
• yarn execution-host-client:pack:verify
• npm publish dry run for 6.0.0-next.0 (152 files)
• git diff --check