Add an archcore import command to bootstrap .archcore/ from CLAUDE.md

[ivklgn]

Problem

There's no supported way to bootstrap .archcore/ from a repository's existing agent-context files (e.g. CLAUDE.md). New users must author documents from scratch, which raises the barrier to adoption.

Expected result

A new archcore import --from <file> command reads a CLAUDE.md and creates a single doc-type document via the existing creation path, with a dry-run preview before writing.

Impact

Lowers onboarding friction — a team can seed .archcore/ from context it already maintains in one command, improving the first-run experience and adoption.

Scope

• Add an import cobra command with a --from flag and a dry-run preview.
• Parse CLAUDE.md and create a single doc.
• Handle "already initialized" and "file not found" cleanly.
• Table-driven tests using t.TempDir().

Files: cmd/import.go (new) + cmd/import_test.go, registered in cmd/root.go

Acceptance: archcore import --from CLAUDE.md previews, then creates one document.