refactor(core): move service integrations to owner crate

[limityan]

Summary

• Move service integration ownership into bitfun-services-integrations feature groups while keeping bitfun-core legacy facade paths available.
• Add decomposition boundary checks and a built-in tool registry manifest baseline so moved ownership does not drift back into bitfun-core.
• Update decomposition guardrails and planning docs for the current P2/P3 boundary, including the latest main branch mode-scoped subagent visibility changes.

Behavior and Risk

• No product behavior change is intended.
• Git service implementation is owned by bitfun-services-integrations; bitfun-core::service::git::* remains a compatibility facade.
• Remote SSH, MCP, and announcement changes only move pure contracts/types or stable helpers; high-coupling runtimes remain core-owned.
• MCP dynamic tools, remote-connect, miniapp runtime, function-agent runtime, and tool registry/provider migration remain explicitly deferred.

Verification

• node scripts/check-core-boundaries.mjs
• BITFUN_BOUNDARY_CHECK_SELF_TEST=1 node scripts/check-core-boundaries.mjs
• git diff --check fork/main..HEAD
• cargo --config "http.proxy=''" test --locked -p bitfun-services-integrations --features product-full
• cargo --config "http.proxy=''" check --locked -p bitfun-core --features product-full

Commit Shape

This PR is intentionally limited to 3 commits:

1. refactor(core): move service integrations to owner crate
2. test(core): enforce decomposition boundaries
3. docs(core): update decomposition guardrails