refactor: add runtime owner boundaries

[limityan]

Summary

• Moves remote workspace/session/file-projection owner contracts into bitfun-runtime-ports, while keeping bitfun-services-integrations::remote_connect compatibility re-exports for existing callers.
• Adds bitfun-agent-runtime as the first real Agent Runtime SDK owner crate for scheduler/background-delivery pure decisions; concrete scheduler/session lifecycle remains in bitfun-core.
• Updates runtime boundary checks, AGENTS guidance, architecture docs, and the active plan so the implementation state does not overclaim unfinished migration work.

Design / behavior impact

• No product commands, UI, default feature set, release scripts, or build-shape behavior changed.
• Remote SSH / relay / terminal pre-warm / session restore / scheduler submit execution stays core-owned; this PR only moves stable contract ownership and keeps old import paths available.
• bitfun-agent-runtime currently owns only background delivery decision facts. Session manager, prompt loop, subagent registry, post-turn hooks, and concrete scheduler lifecycle are explicitly still not migrated.
• No design-document deviation was required. The main documented consequence is that future runtime migration work must treat bitfun-agent-runtime as an actual owner crate, but only for the narrow decision surface introduced here.

Verification

• cargo test -p bitfun-runtime-ports remote_
• cargo test -p bitfun-runtime-services
• cargo test -p bitfun-agent-runtime
• cargo test -p bitfun-services-integrations --features remote-connect --test remote_connect_contracts remote_connect_file
• cargo test -p bitfun-services-integrations --features remote-connect --test remote_connect_contracts remote_connect_workspace_response_helpers_own_wire_shape
• cargo test -p bitfun-core --features product-full cancelled_reply_is_skipped_only_when_suppressed
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene
• git diff --check gcwing/main...HEAD
• cargo check --workspace (passed with existing parse_clipboard_path_segments dead_code warning in bitfun-desktop)
• GitHub Actions CI run #1859: passed (Frontend Build, Rust checks on Windows, Ubuntu, and macOS)

Relates to #970. This PR does not close the architecture migration issue.