fix(subagent-driven-development): namespace SDD scratch workspace by plan

[natem1095]

Problem

scripts/task-brief and scripts/review-package write SDD's scratch
artifacts to a fixed per-repo path (.superpowers/sdd/task-N-brief.md,
and — per SKILL.md's Durable Progress section — .superpowers/sdd/progress.md),
scoped only by task number, not by plan/epic/session.

Two unrelated orchestrator sessions running subagent-driven-development
concurrently in the same working tree, each numbering their tasks from 1,
collide on the same filenames and silently overwrite each other's briefs,
reports, and progress ledger. Observed live: a task-1-report.md at that
path was overwritten by two unrelated task sequences before a third
session's own Task 1 implementer overwrote it a third time.

Fix

• sdd-workspace gains an optional --ns NAMESPACE that scopes the
  returned directory to a subdirectory of .superpowers/sdd/. Omitting it
  preserves the exact prior behavior (flat .superpowers/sdd/), so this is
  fully backward compatible.
• task-brief derives NAMESPACE automatically from PLAN_FILE's
  basename (minus extension) and passes --ns internally — no change to
  task-brief's own call sites.
• review-package accepts --ns NAMESPACE (parsed as a flag so it can
  appear anywhere relative to BASE/HEAD/OUTFILE, preserving the
  existing positional contract).
• SKILL.md now defines NAMESPACE once (right after reading the plan)
  in the Durable Progress section, and threads it through every
  review-package invocation and the progress ledger path
  (scripts/sdd-workspace --ns NAMESPACE), so briefs, reports, review
  packages, and the ledger all land in the same per-plan subdirectory.

Testing

Extended tests/claude-code/test-sdd-workspace.sh with a new scenario:
two unrelated plans (plan-a.md, plan-b.md), each with a "Task 1",
run through task-brief in the same repo — asserts the two briefs land
in different files with the correct per-plan content, that
sdd-workspace --ns resolves the namespaced subdirectory, and that
review-package --ns writes under it too.

$ bash tests/claude-code/test-sdd-workspace.sh
=== Test: sdd-workspace ===
  [PASS] prints <repo-root>/.superpowers/sdd
  [PASS] self-ignoring .gitignore created with '*'
  [PASS] workspace invisible to git status
  [PASS] git add -A does not stage the workspace
  [PASS] task-brief writes its brief under the workspace
  [PASS] review-package writes its diff under the workspace
  [PASS] task-brief namespaces unrelated plans' Task 1 briefs into different files
  [PASS] each namespaced brief holds only its own plan's content
  [PASS] sdd-workspace --ns resolves the namespaced subdirectory
  [PASS] review-package --ns writes its diff under the namespaced subdirectory
  [PASS] linked worktree resolves its own distinct workspace
  [PASS] worktree workspace invisible to git status

PASS

All 8 pre-existing assertions still pass unmodified, confirming the
no---ns path is unchanged.