refactor(agent-runtime): own prompt loop facts

[limityan]

Summary

• Move prompt-loop owner facts into bitfun-agent-runtime: user-context policy, listing section rendering, and prepended reminder ordering.
• Keep bitfun-core prompt builder behavior intact through compatibility re-exports while concrete prompt assembly, workspace/remote context IO, and prompt cache coordination remain core-owned.
• Add owner-contract tests and boundary checks so these prompt facts do not drift back into core.
• Update plan/completed and module AGENTS docs with the migrated scope.
• Clean up architecture/design docs so they stay stable design references rather than PR progress logs; completion state remains tracked through the plan docs and issue.

Impact and Boundaries

• No product command, UI, default feature, tool exposure, permission, scheduler lifecycle, or prompt assembly behavior is intentionally changed.
• Existing core import paths remain compatible.
• This is a runtime owner migration, not a user-visible behavior change.
• Architecture/design docs should only change again if target layering, interface ownership, behavior boundaries, or major risk assumptions need correction.

Validation

• cargo test -p bitfun-agent-runtime
• cargo test -p bitfun-core --features product-full prompt -- --nocapture
• cargo check -p bitfun-core --features product-full
• cargo check -p bitfun-core --no-default-features
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene
• git diff --check

Refs #970