feat(tools): transactional ShellExecutor with snapshot+rollback (#2414)

[bug-ops]

Summary

• Add opt-in snapshot+rollback to ShellExecutor for atomic file operations (closes research(tools): transactional ShellExecutor — snapshot+rollback for atomic file operations (arXiv:2512.12806) #2414)
• New transaction.rs module: TransactionSnapshot captures filesystem state before write commands and restores on failure
• Shell redirection targets (>, >>, 2>, 2>>, &>, &>>) are parsed and included in snapshot scope
• Symlinks are detected and skipped during snapshot to prevent traversal outside sandbox
• rollback() attempts all file restores before returning error (no early-exit on partial failure)

New config fields ([tools.shell])

transactional = false              # opt-in snapshot+rollback
transaction_scope = []             # glob patterns; empty = all affected paths
auto_rollback = false              # restore snapshot on exit code >= 2
auto_rollback_exit_codes = []      # explicit exit codes that trigger rollback
snapshot_required = false          # abort execution if snapshot creation fails

Integration points

• --init wizard: prompts for transactional and auto_rollback
• --migrate-config: Step 6 handles new optional fields
• TUI: ToolEvent::Rollback surfaced as status indicator via tui_bridge
• Audit: new AuditResult::Rollback variant logged to audit trail

Test plan

• 903 tests pass (18 new covering: snapshot/rollback, redirection parsing, scope filtering, exit code semantics, snapshot failure modes, symlink skipping, partial rollback error collection)
• cargo +nightly fmt --check clean
• cargo clippy -- -D warnings clean

Follow-up issues to file after merge

• Snapshot size limit (max_snapshot_bytes config field) — security audit MEDIUM
• Symlink rollback write risk — security audit MEDIUM (symlinks currently skipped at capture; rollback is safe)