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/execute-phase.md

@@ -456,8 +456,8 @@ Apply the same "incomplete" filtering rules as earlier:
 
 Selected wave finished successfully. This phase still has incomplete plans, so phase-level verification and completion were intentionally skipped.
 
-/gsd:execute-phase {phase}                # Continue remaining waves
-/gsd:execute-phase {phase} --wave {next}  # Run the next wave explicitly
+/gsd:execute-phase {phase} ${GSD_WS}                # Continue remaining waves
+/gsd:execute-phase {phase} --wave {next} ${GSD_WS}  # Run the next wave explicitly
 ```
 
 **If no incomplete plans remain after the selected wave finishes:**
@@ -602,7 +602,7 @@ grep "^status:" "$PHASE_DIR"/*-VERIFICATION.md | cut -d: -f2 | tr -d ' '
 |--------|--------|
 | `passed` | → update_roadmap |
 | `human_needed` | Present items for human testing, get approval or feedback |
-| `gaps_found` | Present gap summary, offer `/gsd:plan-phase {phase} --gaps` |
+| `gaps_found` | Present gap summary, offer `/gsd:plan-phase {phase} --gaps ${GSD_WS}` |
 
 **If human_needed:**
 
@@ -679,15 +679,15 @@ Items saved to `{phase_num}-HUMAN-UAT.md` — they will appear in `/gsd:progress
 ---
 ## ▶ Next Up
 
-`/gsd:plan-phase {X} --gaps`
+`/gsd:plan-phase {X} --gaps ${GSD_WS}`
 
 <sub>`/clear` first → fresh context window</sub>
 
 Also: `cat {phase_dir}/{phase_num}-VERIFICATION.md` — full report
-Also: `/gsd:verify-work {X}` — manual testing first
+Also: `/gsd:verify-work {X} ${GSD_WS}` — manual testing first
 ```
 
-Gap closure cycle: `/gsd:plan-phase {X} --gaps` reads VERIFICATION.md → creates gap plans with `gap_closure: true` → user runs `/gsd:execute-phase {X} --gaps-only` → verifier re-runs.
+Gap closure cycle: `/gsd:plan-phase {X} --gaps ${GSD_WS}` reads VERIFICATION.md → creates gap plans with `gap_closure: true` → user runs `/gsd:execute-phase {X} --gaps-only ${GSD_WS}` → verifier re-runs.
 </step>
 
 <step name="update_roadmap">
@@ -801,10 +801,10 @@ Read and follow `~/.claude/get-shit-done/workflows/transition.md`, passing throu
 ```
 ## ✓ Phase {X}: {Name} Complete
 
-/gsd:progress — see updated roadmap
-/gsd:discuss-phase {next} — discuss next phase before planning
-/gsd:plan-phase {next} — plan next phase
-/gsd:execute-phase {next} — execute next phase
+/gsd:progress ${GSD_WS} — see updated roadmap
+/gsd:discuss-phase {next} ${GSD_WS} — discuss next phase before planning
+/gsd:plan-phase {next} ${GSD_WS} — plan next phase
+/gsd:execute-phase {next} ${GSD_WS} — execute next phase
 ```
 
 Only suggest the commands listed above. Do not invent or hallucinate command names.