fix(agent-runtime): replace removed Pi harness APIs

[vastsa]

Summary

• Replace calls to removed experimental pi-agent-core harness helpers with runtime-owned adapters for context projection, message conversion, estimates, compaction, and prompt templates.
• Keep pi-agent-core for its stable Agent loop and types, and preserve Rust host-core as the only durable session owner.
• Document pi-coding-agent as a transitional dependency with a removal path, plus the separate pi-durable assessment.
• Snapshot split sidecar bundles correctly in the offline hosted-search E2E harness.

Specs and scenarios

• Updated runtime and architecture specs, ADRs, the decisions log, and Chinese locale mirrors.
• Relevant scenarios: compaction coverage in context-compaction.test.mjs; offline hosted-search continuation and restore scenarios in pnpm test:e2e:hosted-search.
• The full provider/UI context-compaction journey (E2E-164) remains Draft in the repository plan.

Validation

• Agent Runtime typecheck/build and package bundle passed.
• Targeted Agent Runtime tests: 13 files, 470 tests passed.
• Desktop context-compaction contract: 13/13 passed.
• Offline hosted-search sidecar E2E: 7/7 passed on this commit; no live provider used.
• pnpm lint, pnpm docs:check, pnpm check:pr-base, and git diff --check passed.