feat: add backlog init command

[tkstang]

feat: add backlog init command

Summary

This adds an explicit oat backlog init command for repositories that have OAT installed but do not yet have the local file-backed backlog structure. The command scaffolds the canonical backlog directories and starter files, preserves curated content on rerun, and now keeps the empty backlog directories versionable across git round-trips. Because this was delivered in quick mode, the PR is grounded in discovery.md, plan.md, and implementation.md rather than a separate spec/design pair.

Goals / Non-Goals

• Goal: add a first-class CLI entry point for creating .oat/repo/reference/backlog/, items/, archived/, index.md, and completed.md.
• Goal: keep the scaffold idempotent so reruns do not clobber curated backlog content.
• Goal: prove the scaffold works with existing backlog flows, especially oat backlog regenerate-index.
• Non-goal: change oat-pjm-* skills or add automatic backlog initialization behavior.
• Non-goal: introduce any remote project-management integration behavior.

Changes

• Added packages/cli/src/commands/backlog/init.ts with reusable scaffold logic and starter content for index.md and completed.md.
• Wired oat backlog init into the backlog command group with --backlog-root, text output, and JSON output in packages/cli/src/commands/backlog/index.ts.
• Added helper-level regression coverage in packages/cli/src/commands/backlog/init.test.ts.
• Added compatibility coverage in packages/cli/src/commands/backlog/regenerate-index.test.ts, including a commit/clone round-trip that verifies the scaffold remains usable without rerunning init.
• Added command-level coverage in packages/cli/src/commands/backlog/index.test.ts for default root resolution, --backlog-root, JSON output, and exit-code behavior.
• Seeded .gitkeep placeholders so empty items/ and archived/ directories survive normal git workflows.

Verification

• pnpm --filter @oat/cli test -- src/commands/backlog/init.test.ts
• pnpm --filter @oat/cli test -- src/commands/help-snapshots.test.ts
• pnpm --filter @oat/cli test -- src/commands/backlog/init.test.ts src/commands/backlog/regenerate-index.test.ts src/commands/help-snapshots.test.ts && pnpm type-check
• pnpm --filter @oat/cli test -- src/commands/backlog/init.test.ts src/commands/backlog/regenerate-index.test.ts
• pnpm --filter @oat/cli test -- src/commands/backlog/index.test.ts src/commands/backlog/init.test.ts src/commands/backlog/regenerate-index.test.ts
• pnpm test
• pnpm lint
• pnpm type-check
• pnpm build

Reviews

Scope	Type	Status	Date	Artifact
final	code	passed	2026-03-20	reviews/archived/final-review-2026-03-20-v2.md

References

• Discovery: discovery.md
• Plan: plan.md
• Implementation: implementation.md