fix(core): preserve queue after provider failure

[kitlangton]

Summary

• distinguish continued, completed, and durably failed provider-turn outcomes
• stop the current Session drain after a projected terminal provider failure instead of promoting pending input
• preserve queued work for a later explicit resume and document the behavior

Testing

• bun run test test/session-runner.test.ts in packages/core (81 passed)
• bun run typecheck in packages/core
• pre-push workspace typecheck (23 passed)
• git diff --check

Review note

The parallel simplify reviewers could not start because their local session database is missing the replacement_seq column.