Let Skywalker implement bounded work as a judgment call

[TheGreatAxios]

Summary

• Skywalker primary sessions mount write/edit/delete path tools.
• Tiny, single-file, or one-route bounded edits may be done on the parent; spawn remains the default for substantial, multi-file, parallel, or specialist work (hard cap 4 workers).
• Greybeard and other nested orchestrators stay write-free. Shell file-writes stay denied.

Verification

• bun run typecheck and bun run build pass
• bun run test: 4984 pass; 4 failures are pre-existing (TUI shell flake, two grok-4.6 cycle assertions vs xhigh, local vendor/.DS_Store scope). 6629 remount tests pass

Fixes CL-6629

[linear-code]

CL-6629