SDD progress ledger: single repo-level path gets clobbered by concurrent SDD sessions

[rz1989s]

Repro (hit for real, 2026-09-03)

Two SDD sessions ran in this repo in parallel windows:

1. Session A: feat/104-fleet-presentation-redesign (plan 2026-09-03-fleet-presentation-p1, base d9345bb) — wrote its ledger line to .superpowers/sdd/progress.md.
2. Session B (this one): feat/1b-2-fleet-governance-adapter — followed the SDD skill's ledger check (cat .superpowers/sdd/progress.md), saw the file, and then overwrote it with > while initializing its own section. Session A's recovery map was destroyed (restored by hand from session B's scrollback).

Root cause

.superpowers/sdd/progress.md is a single repo-level path. The SDD skill treats it as the durable cross-compaction recovery map, but its identity is not scoped to the session/branch executing the work. Any second concurrent SDD session (different branch, same checkout dir or a sibling worktree of the same repo) reads-and-tramples the first one's ledger.

Secondary hazard observed in the same incident: two sessions sharing one working directory can also pull the branch out from under each other via git checkout (session B switched branches while A's tree state was live).

Expected

• Ledger writes are append/merge-safe across concurrent sessions (or ledger path is per-branch, e.g. .superpowers/sdd/progress.<branch>.md, resolved from git branch --show-current).
• The skill's ledger-init step should detect a foreign/unrelated existing ledger and MERGE (new section + a tombstone note), never >.

Suggested fix direction

• scripts/sdd-workspace (or a small sdd-ledger helper) owns ledger init/append: resolve per-branch filename, create-if-absent, append sections. The skill text references the fixed path in multiple places and would need the pointer updated.

Environment

• armory-fleet @ aa0dbef (both sessions), SDD skill via superpowers, macOS, pi sessions in the same checkout directory.