Support .cursorrules and copilot-instructions in archcore import

[ivklgn]

Problem

The initial archcore import MVP only reads CLAUDE.md, but teams keep agent context in other formats too (.cursorrules, .github/copilot-instructions.md).

Expected result

archcore import detects the source format and imports .cursorrules and copilot-instructions as single documents, in addition to CLAUDE.md.

Impact

Broadens onboarding coverage to the most common agent-context formats.

Scope

• Add source-format detection (by filename/path).
• Add parsers for .cursorrules and copilot-instructions.
• Cover each format with tests.

Files: cmd/import.go (+ tests)

Acceptance: all three formats import as single documents with format-appropriate handling.

Depends on the initial archcore import command.