fix(skywalker): spawn one focused task per worker

[TheGreatAxios]

Summary

• Skywalker and primary orchestration prompts assign one focused task per spawned worker; fan-out width follows independent lanes (one lane per PR/path/ownership).
• Dispatch brief contract (intent, success_criteria, do_not, report_focus) stays tight; spawn_agent copy and orchestration docs match.

Verification

• bun run typecheck, bun run build, and bun run lint pass
• Targeted prompt tests pass (src/agent/directors/skywalker/package.test.ts, src/agent/prompts.test.ts, src/prompts.test.ts)
• bun run check ran the full suite: 7823 pass / 1 fail — background shell registry > collect with an aborted signal releases as running without killing the child (pgrep flake in src/shell/background-shell.test.ts, unrelated). Re-run of that file: 17 pass / 0 fail

Fixes CL-8746

[linear-code]

CL-8746