Polish docs, add repo navigation, and extend legacy reference snapshots

[mdheller]

Summary

• add a docs index, development guide, repo-role note, and PR template
• extend the legacy v2.3 in-repo reference archive with additional raw snapshots
• keep the work based on the current live main after the merged scaffold PRs
• avoid touching bootstrap business logic or unfrozen boot/auth semantics

Why

The repository already moved beyond handoff-only state via the merged scaffold work. This PR is the next small polish tranche: it improves repo navigation and contributor ergonomics, and makes the legacy lineage archive more complete for future controlled porting.

Included

• docs/INDEX.md
• docs/DEVELOPMENT.md
• docs/REPO_ROLE.md
• .github/PULL_REQUEST_TEMPLATE.md
• docs/legacy/prophet_cli_v2_3/raw/root.go.txt
• docs/legacy/prophet_cli_v2_3/raw/main.go.txt

Explicit boundaries

• no bootstrap business logic added here
• no in-place semantic changes to BootReleaseSet / ConfigSource / TokenDoor ownership
• no boot/auth/enrollment semantics invented here
• this remains a facade-only repository