fix(evolution): restore snapshots without deleting live trees first

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Summary

• Evolution rollback deleted live memory/evolution and _evolved skill trees before copying the snapshot back. A copy failure, crash, or kill after remove_dir_all destroyed current evolution memory and evolved skills even though the snapshot still existed. Auto-rollback (check_auto_rollback) uses this path, so recovery could wipe the data it is supposed to restore.
• Restore now copies the snapshot tree into a sibling temp directory, then rename-swaps it into place. If the copy fails, the live destination is left intact. A failed final rename attempts to move the backup back.

Task Linkage

• Task ID: TASK-2026-071
• Task folder: tasks/TASK-2026-071-evolution-restore-atomic/

Injected Specs

• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/testing-policy.md (required for any code change)
• spec/docs-sync.md (if behavior/docs/env/commands changed)
• Additional injected specs followed: spec/verification-integrity.md, spec/task-artifact-language.md, spec/rust-conventions.md.

Validation Evidence

• Commands executed:
  • rustup update stable && rustup default stable → rustc 1.97.1 / cargo 1.97.1
  • cargo fmt --check
  • cargo clippy --all-targets -- -D warnings
  • cargo test -p skilllite-evolution --lib -- extended_snapshot_restores_memory_and_skills restore_extended_snapshot_removes_live_files_absent_from_snapshot replace_dir_from_snapshot_leaves_destination_intact_when_copy_fails
  • cargo test
  • python3 scripts/validate_tasks.py
• Key results:
  • rustfmt: exit 0
  • clippy: Finished dev profile with -D warnings, exit 0
  • targeted tests: 3 passed; 0 failed including copy-failure integrity and extra-file replacement
  • workspace cargo test: all crate results 0 failed
  • task validation: Task validation passed (71 task directories checked).

Regression Scope

• Areas likely affected:
  • Auto-rollback (check_auto_rollback → restore_extended_snapshot)
  • Manual/CLI restore of evolution memory and _evolved skills
• Explicit non-goals:
  • Prompt-file in-place copy
  • Snapshot create/prune ordering
  • ChatSession workspace chat-root alignment
  • Concurrent sessions.json writes

Docs Sync (EN/ZH)

• Not needed
• Updated EN + ZH docs
• Files:
  • none (internal restore correctness; no command/flag/env change)

Review Checklist

• Acceptance criteria in tasks/TASK-2026-071-evolution-restore-atomic/TASK.md satisfied (or explicitly deferred)
• tasks/TASK-2026-071-evolution-restore-atomic/STATUS.md updated with latest progress
• tasks/TASK-2026-071-evolution-restore-atomic/REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date