feat(backups): automatic backup system with TUI restore flow

[davidarce]

Summary

• Add an automatic backup system for devrune.yaml: every mutation (install / sync / upgrade / TUI manifest writes) creates a timestamped snapshot first, with FIFO rotation at 5 backups per project.
• Provide a TUI Backups menu entry that lists snapshots newest-first, shows a unified diff against the current manifest, asks for confirmation, takes a pre-restore backup of the live state, replaces devrune.yaml atomically, and re-runs install to materialize artifacts.
• Centralize all manifest writes through writeManifestSafe (backup + atomic rename) so no path can mutate devrune.yaml without first creating a recovery point.

Implementation highlights

• internal/backup/atomic.go — WriteFileAtomic (write to .tmp, Sync, os.Rename) for crash-safe writes.
• internal/backup/backup.go — CreateBackup / ListBackups with timestamped filenames, FIFO rotation, idempotent .gitignore management.
• internal/diff/diff.go — minimal line-diff used by the restore preview screen (no external deps).
• internal/cli/manifest_io.go — writeManifestSafe shared by init, sync, menu paths, advisor flows, and model selector.
• internal/tui/steps/backups.go — full restore flow (list → empty state → diff → confirm → in-progress → success/error). Install failure surfaces an error and leaves the restored manifest on disk (no auto-revert).
• Tests: atomicity, rotation, reentrancy, hook integration, restore flow with injectable installFn (847 new test lines across the four packages).

Test plan

• go build ./... clean
• go test ./internal/backup/... ./internal/diff/... ./internal/cli/... ./internal/tui/steps/... green
• Manual smoke: devrune init creates .devrune/backups/, second init rotates the previous snapshot
• Manual smoke: TUI Backups entry shows snapshots, diff preview matches, confirmation triggers pre-restore backup + re-install
• Manual smoke: simulate disk-full on backup creation → operation aborts with clear error, devrune.yaml untouched

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