Add section-aware splitting to archcore import

[ivklgn]

Problem

Importing a source file as one large document loses structure. Ideally an import splits by heading into multiple correctly-typed documents (e.g. "Build commands" → a doc, "Architecture decision" → an adr). This needs a design decision on heuristic vs. LLM-assisted classification.

Expected result

archcore import can split a multi-section source into several typed documents, shown as a multi-document preview and created on confirm.

Impact

Produces a well-structured .archcore/ on import instead of one monolithic blob, so the imported context is immediately useful.

Scope

• Agree the splitting strategy (start heuristic: split on ## headings; classify by keyword) — note classification may be better delegated to the agent via MCP than implemented in Go.
• Emit a multi-document preview and create the set on confirm.
• Document the mapping rules; add fixtures + tests.

Files: cmd/import.go (+ tests)

Acceptance: a multi-section source imports into several correctly-typed documents.

Status: needs design. Depends on the initial archcore import command.