Move integration runtime helpers out of core

[limityan]

Summary

• Update the core decomposition docs to compress the remaining work into five reviewable PRs, with lower-risk owner-crate moves earlier and high-risk runtime/feature-shape decisions deferred behind explicit guardrails.
• Move remote-SSH workspace registry ownership into bitfun-services-integrations::remote_ssh::RemoteWorkspaceRegistry while keeping bitfun-core as the product assembly wrapper for local assistant path guarding and SSH/file/terminal manager handles.
• Move announcement state persistence into bitfun-services-integrations::announcement::AnnouncementStateStore, preserving the existing core PathManager constructor and error mapping through a compatibility wrapper.
• Add boundary checks and owner-crate contract tests so future refactors do not silently move persistence or registry logic back into core.

Functional impact

This is intended to be behavior-preserving. The PR does not move SSH channel/SFTP/PTY execution, remote-connect runtime, MCP dynamic tools, ToolUseContext, concrete tool implementations, miniapp/function-agent runtime, agent registry, or product feature assembly.

Core still performs the local assistant workspace guard before remote path resolution. Remote workspace ambiguous-root handling, active connection hinting, legacy state snapshot shape, announcement load fallback, and announcement save semantics are covered by tests after the ownership move.

Validation

• node scripts/check-core-boundaries.mjs
• $env:BITFUN_BOUNDARY_CHECK_SELF_TEST='1'; node scripts/check-core-boundaries.mjs
• rustfmt --edition 2021 --check <touched rust files>
• git diff --check -- <pr files>
• cargo test -p bitfun-services-integrations --features "remote-ssh announcement file-watch"
• cargo test -p bitfun-core remote_workspace -- --nocapture
• cargo check -p bitfun-services-integrations --features product-full
• cargo test -p bitfun-services-integrations --features product-full
• cargo tree -p bitfun-services-integrations --no-default-features --depth 1 --edges features
• cargo check --workspace
• cargo test --workspace

Note: the workspace checks still report the existing bitfun-cli unused Read import warning in src/apps/cli/src/ui/theme.rs; this PR does not touch that file.