DOCS: Add FILE_INDEX.md - framework navigation map (English adaptation) #5

Origin

Spawned from #4 (evaluation of local-only framework files).

What

FILE_INDEX.md exists locally in multiagentsystem_v4 as a navigation map of all framework files - their paths, purposes, and cross-references. It was excluded from the initial push because it was written in Russian and referenced local machine paths.

Why include it

This is a framework-level onboarding doc, not a local runtime artifact. New contributors and agents need a single map of "what file does what" - this fills that gap and nothing in current canonical repo covers it.

Work needed

1. Read local FILE_INDEX.md content
2. Translate to English
3. Replace any local machine paths with canonical GitHub paths (e.g., /mnt/c/... ? relative repo paths)
4. Remove or generalize any multiagentsystem_v4-specific references ? use multiagentsystem
5. Open PR targeting main branch

Acceptance criteria

• English only, no Russian text
• No local machine paths (/mnt/, C:\Users\...)
• References match files actually present in canonical repo
• Reviewed against current repo structure - all referenced files exist