refactor(agent-runtime): own event facts

[limityan]

Scope

• Move portable runtime event facts into bitfun-agent-runtime:
  • FinishReason wire labels
  • session-state event labels
  • turn-outcome event fact mapping used by agent-session reply policy
• Keep bitfun-core as compatibility facade / concrete adapter only:
  • execution types re-export FinishReason
  • event types map concrete SessionState to portable facts before formatting
  • scheduler delegates turn-outcome fact mapping to bitfun-agent-runtime
• Extend core boundary checks so these facts cannot drift back into core.
• Update plan/completed docs and module AGENTS only; stable architecture/design docs are unchanged because the target design did not change.

Boundary / Behavior

• No product command, UI, default feature, release/fast-build script, or product capability changes.
• No scheduler lifecycle, event delivery, post-turn hook, session manager, or concrete prompt assembly migration in this PR.
• No new dependency from bitfun-agent-runtime back to bitfun-core or concrete service crates.

Refs #970

Verification

• pnpm run fmt:rs
• cargo test -p bitfun-agent-runtime --test events_contracts -- --nocapture
• cargo test -p bitfun-core cancelled_reply_is_skipped_only_when_suppressed -- --nocapture
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene