feat(phases): scoutctl phases backport — add --date flag and write '<!-- backported {date} -->' provenance marker

[jordanrburger]

Context

docs/superpowers/specs/2026-06-16-scoutctl-phases-backport-design.md specifies scoutctl phases backport (reverse-map vault brain-file edits into phases/ fragments). The core shipped in #139 (engine/scout/scripts/phase_backport.py + CLI + 18 passing tests), but a spec-vs-code audit (2026-06-24) found two spec items still missing.

Gaps to close

• --date YYYY-MM-DD flag — spec requires overriding the marker date (default: today); the flag is not implemented
• Backport provenance marker — spec requires writing <!-- backported {date} --> to each retemplatized section on --apply. Edits are currently written silently with no marker, so backported sections aren't traceable.

Reference

• Spec: docs/superpowers/specs/2026-06-16-scoutctl-phases-backport-design.md
• Core implementation: engine/scout/scripts/phase_backport.py; CLI in engine/scout/cli.py (_register_phases)

_{Filed from a spec-implementation audit run via Claude Code.}