feat: resume delegated tasks by thread id

[qiunai]

Summary

• add codex-companion task --resume-thread <thread-id> to resume an exact persisted Codex task thread
• route the explicit thread id through foreground and background task execution
• cover the exact-thread resume path with a runtime test

Motivation

Claude/Codex collaboration flows can have multiple delegated task threads active at once. --resume-last is useful for simple cases, but exact thread routing prevents cross-thread continuation when several Claude conversations are driving Codex tasks concurrently.

Validation

• node --check plugins/codex/scripts/codex-companion.mjs
• npm test

[qiunai]

Updated this PR to also expose explicit task sandbox routing for delegated workflows:\n\n- added codex-companion task --full-access as a shorthand for --sandbox danger-full-access\n- added --sandbox <read-only|workspace-write|danger-full-access> for explicit task sandbox selection\n- covered full-access forwarding through thread start/resume in runtime tests\n\nThis is needed alongside --resume-thread because exact thread resume identifies the Codex thread, but the task launcher still needs to pass the intended filesystem sandbox for implementation/rework turns.