fix: scope SDD artifacts by plan identity

[luochen211]

Summary

• scope SDD workspace directories by plan identity when a plan is available
• route task briefs, review packages, and progress-ledger instructions through the same plan-scoped workspace
• add regression coverage for concurrent same-basename plans in one worktree

Why

Multiple SDD runs in the same working tree currently write briefs, reports, review packages, and progress state into the same flat .superpowers/sdd directory. That can overwrite artifacts and make compaction recovery resume the wrong plan.

Fixes #1936
Related #1816
Related #1888

Testing

• tests/claude-code/test-sdd-workspace.sh
• tests/claude-code/test-worktree-path-policy.sh
• bash tests/shell-lint/test-lint-shell.sh
• bash -n skills/subagent-driven-development/scripts/sdd-workspace skills/subagent-driven-development/scripts/task-brief skills/subagent-driven-development/scripts/review-package tests/claude-code/test-sdd-workspace.sh
• git diff --check

Notes

• tests/claude-code/run-skill-tests.sh --test test-sdd-workspace.sh and tests/claude-code/test-subagent-driven-development.sh did not run through their wrapper on this macOS host because GNU timeout is not installed; the targeted SDD workspace test was run directly and passed.