feat(shell-job): add isolated shell job service

[kitlangton]

Summary

• add an isolated ShellJob Effect service for starting, waiting on, reading, and killing background shell jobs
• add JobID and disk-backed spool metadata/output under .opencode/jobs/<job-id>/
• add focused shell-job tests covering output capture, incremental reads, failure, kill, and instance disposal

Notes

• this PR intentionally isolates only the shell-job runtime work and its tests
• prompt/tool integration is left for follow-up work

Testing

• bun run test test/shell-job/shell-job.test.ts in the source workspace
• the isolated PR worktree itself did not resolve the local Bun preload package, so I did not rely on that duplicate worktree run