docs(io): document portable-v2 as the primary project movement workflow

[DecisionNerd]

Scope

Complete #740 acceptance criterion 9 with user-facing and API-linked documentation for portable project v2 as GraphForge’s primary project movement workflow.

This is a documentation-only close blocker. It must not duplicate or modify #851 binding implementation.

Acceptance criteria

• A user guide covers preview, expanded and bundled export, full verification, clean complete import, selective sharing, local/air-gapped transfer, and digest-pinned OCI promotion.
• Rust API and CLI entry points are mapped to the same Rust-owned workflow, with links to the normative architecture/specification.
• Security guidance separates integrity, compatibility, authenticity, package identity, and transport identity and documents safe credential handling.
• Documentation explicitly prohibits copying or archiving live .graphforge state as interchange and committing graph/source/package/materialized data to Git.
• Documentation navigation exposes the guide and repository documentation validation passes.

Relationships

Narrow sub-issue and blocker of #740. #740 remains the canonical portable-v2 close gate.