chore(sc-43976): design + runbook to move git history to archive repo

[yodem]

Summary

• Adds design doc + one-shot migration runbook for splitting Sefaria-Export into a lean main repo + a new Sefaria-Export-Archive holding the full historical commit graph (resolves sc-43976).
• Adds a "Cloning" section to the README pointing users at --depth 1 and the (forthcoming) archive repo.
• Adds project-scope Shortcut MCP via .mcp.json.example (real .mcp.json is gitignored — contains an API token).

This PR does not perform the destructive force-push. Phase 1 and Phase 2 scripts in scripts/migration/ are run by a maintainer with admin rights on the Sefaria org, after this PR is reviewed and merged.

What's in here

• docs/superpowers/specs/2026-05-06-archive-git-history-design.md — full design: two-repo split, alternatives considered, 4-phase migration plan, risks, verification, recovery procedure.
• scripts/migration/01_create_archive.sh — mirror-clone source repo to archive (non-destructive). Tags pre-migration-master for recovery.
• scripts/migration/02_orphan_master.sh — replace master with single orphan commit. Verifies the new tree hash matches the pre-migration tree before pushing. Two confirm prompts. Uses --force-with-lease.
• scripts/migration/README.md — runbook overview, ordering, prerequisites, recovery.
• README.md — new "Cloning" section.
• .gitignore / .mcp.json.example — keep the Shortcut API token out of the public repo.

Test plan

• Review the design doc — flag any concerns about hash rewriting, fork breakage, or branch protection before the cutover is scheduled.
• Skim both migration scripts; verify the prompts and the tree-hash sanity check.
• Decide a freeze date and notify open-PR / fork owners.
• Confirm next `generate-books-json` workflow run is green before the cutover.
• (Post-merge, manual) create empty `Sefaria/Sefaria-Export-Archive`, run `01_create_archive.sh`, verify on GitHub, archive the repo.
• (Post-merge, manual) relax branch protection, run `02_orphan_master.sh`, restore protection.
• (Post-merge, manual) verify `du -sh .git` on a fresh clone is well under 100 MB.

🤖 Generated with Claude Code

[gitvelocity-reviewer]

📊 Code Quality Score: 18/100

46 × 0.4 (Small ESF) = 18.4, rounded to 18

Category	Score	Factors
🔭 Scope	7/20	4 files across docs/scripts/config/README; single subsystem (repo maintenance); no new APIs or external integrations; cross-cutting only within the repo itself
🏗️ Architecture	8/20	Introduces two-repo split pattern with clear migration path; significant structural decision for the project; no code architecture changes; well-reasoned alternatives table
⚙️ Implementation	9/20	Shell scripts with set -euo pipefail, tree-hash sanity check, interactive confirmation prompts, --force-with-lease, pre-migration SHA recording; non-trivial defensive engineering for runbooks
⚠️ Risk	10/20	Migration itself is high-risk (force-push, history rewrite) but this PR is design/runbook only; risk well-documented with mitigations; multiple abort points in scripts; recovery path documented
✅ Quality	10/15	Comprehensive design doc with alternatives table, risk table, verification steps, recovery path; good inline comments in scripts; no tests (appropriate for runbook); minor gaps between design doc and script implementation
🔒 Perf / Security	2/5	--force-with-lease over bare --force is meaningful safety choice; disk space requirements documented; no benchmarks or security threat model needed for this change type

_{Scored by GitVelocity · How are scores calculated?}