[codex] Add async exec job polling

[arslnb]

Summary

Closes #14.

Adds a first-class async exec job model for POST /v1/sandboxes/:id/exec with background: true:

• returns 202 Accepted with a cmd_id, status_url, and logs_url
• adds GET /v1/sandboxes/:id/exec/:cmd_id for status and exit code polling
• adds GET /v1/sandboxes/:id/exec/:cmd_id/logs for captured stdout/stderr snapshots
• persists exec jobs in SQLite and marks in-flight jobs failed after control-plane restart
• keeps idle and pressure standby reapers from parking sandboxes with running exec jobs
• updates TypeScript/Python SDK helpers and docs

Validation

• cargo test -p sandboxd
• cargo test -p sandboxd background_exec_returns_job_status_and_logs
• npm run build in sdk/typescript
• python -m compileall sdk/python/src/workdir
• cargo fmt --check
• git diff --check