feat!: make team mode the default

[Donmclean]

Summary

• make team mode the default for new scaffolds while retaining explicit local mode
• keep durable context Git-visible and move private continuation state under local/
• preserve legacy Init layout/API behavior and fresh-clone hydration
• harden init, doctor, Git/worktree handling, and update publication safety
• document the 0.2 migration and pre-1.0 breaking-release policy
• refresh scaffold descriptions and domain-specific terminology

Breaking change

New ctx init scaffolds use team mode by default. Pass --mode local for the previous whole-folder-private behavior. Existing legacy scaffolds continue to use legacy-local behavior.

Verification

• go test -race ./... -count=1
• go vet ./...
• make smoke
• Windows amd64/arm64 and FreeBSD amd64 package cross-compilation
• semantic-release rule checks
• npm package dry run
• git diff --check