refactor: group the harness and provider commands - #210

Groups the harness-config and catalogue commands under their own command groups, and gives the harness launch and the default-harness setting their own spellings.

What changes

• harness is now a pure command group. Its six config verbs move underneath it: spinloop harness add|remove|apply|unapply|show|export.
• Launching the harness moves off the bare harness onto spinloop harness open.
• Setting and reading the default harness move onto spinloop harness config (--set/--get).
• The two catalogue commands move under a new spinloop provider group: spinloop provider list and spinloop provider init (renamed from init-providers).
• The old top-level spellings no longer parse. Running one fails naming its new home, e.g. "add" moved: run spinloop harness add.

Details

• Shell completion follows the new command tree; the guard test walks that tree, and new scenarios cover the provider and harness group slots, harness open, and harness config --set.
• All 14 affected capability specs are synced to the new spellings and the group-harness-provider-commands change is archived.
• Docs and examples updated to the new spellings.

Addresses #200 (grouping) and #203 (dedicated launch spelling — takes the harness open proposal; the top-level code shortcut is deferred).