feat: workstream namespacing for parallel milestone work

Enable multiple Claude Code instances to work on the same codebase
simultaneously by scoping .planning/ state into workstreams.

Core changes:
- planningDir(cwd, ws?) and planningPaths(cwd, ws?) are now workstream-aware
  via GSD_WORKSTREAM env var (auto-detected from --ws flag or active-workstream file)
- All bin/lib modules use planningDir(cwd) for scoped paths (STATE.md, ROADMAP.md,
  phases/, REQUIREMENTS.md) and planningRoot(cwd) for shared paths (milestones/,
  PROJECT.md, config.json, codebase/)
- New workstream.cjs module: create, list, status, complete, set, get, progress
- gsd-tools.cjs: --ws flag parsing with priority chain
  (--ws > GSD_WORKSTREAM env > active-workstream file > flat mode)
- Collision detection: transition.md checks for other active workstreams before
  suggesting next-milestone continuation (prevents WS A from trampling WS B)
- ${GSD_WS} routing propagation across all 9 workflow files ensures workstream
  scope chains automatically through the workflow lifecycle

New files:
- get-shit-done/bin/lib/workstream.cjs (CRUD + collision detection)
- get-shit-done/commands/gsd/workstreams.md (slash command)
- get-shit-done/references/workstream-flag.md (documentation)
- tests/workstream.test.cjs (20 tests covering CRUD, env var routing, --ws flag)

All 1062 tests passing (1042 existing + 20 new workstream tests).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

get-shit-done/workflows/validate-phase.md

@@ -35,7 +35,7 @@ SUMMARY_FILES=$(ls "${PHASE_DIR}"/*-SUMMARY.md 2>/dev/null)
 
 - **State A** (`VALIDATION_FILE` non-empty): Audit existing
 - **State B** (`VALIDATION_FILE` empty, `SUMMARY_FILES` non-empty): Reconstruct from artifacts
-- **State C** (`SUMMARY_FILES` empty): Exit — "Phase {N} not executed. Run /gsd:execute-phase {N} first."
+- **State C** (`SUMMARY_FILES` empty): Exit — "Phase {N} not executed. Run /gsd:execute-phase {N} ${GSD_WS} first."
 
 ## 2. Discovery
 
@@ -137,14 +137,14 @@ node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" commit "docs(phase-${PHASE}
 ```
 GSD > PHASE {N} IS NYQUIST-COMPLIANT
 All requirements have automated verification.
-▶ Next: /gsd:audit-milestone
+▶ Next: /gsd:audit-milestone ${GSD_WS}
 ```
 
 **Partial:**
 ```
 GSD > PHASE {N} VALIDATED (PARTIAL)
 {M} automated, {K} manual-only.
-▶ Retry: /gsd:validate-phase {N}
+▶ Retry: /gsd:validate-phase {N} ${GSD_WS}
 ```
 
 Display `/clear` reminder.