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/plan-phase.md

@@ -208,9 +208,9 @@ If "Run discuss-phase first":
   does not work correctly in nested subcontexts (#1009). Instead, display the command
   and exit so the user runs it as a top-level command:
   ```
-  Run this command first, then re-run /gsd:plan-phase {X}:
+  Run this command first, then re-run /gsd:plan-phase {X} ${GSD_WS}:
 
-  /gsd:discuss-phase {X}
+  /gsd:discuss-phase {X} ${GSD_WS}
   ```
   **Exit the plan-phase workflow. Do not continue.**
 
@@ -363,7 +363,7 @@ Use AskUserQuestion:
 - header: "UI Design Contract"
 - question: "Phase {N} has frontend indicators but no UI-SPEC.md. Generate a design contract before planning?"
 - options:
-  - "Generate UI-SPEC first" → Display: "Run `/gsd:ui-phase {N}` then re-run `/gsd:plan-phase {N}`". Exit workflow.
+  - "Generate UI-SPEC first" → Display: "Run `/gsd:ui-phase {N} ${GSD_WS}` then re-run `/gsd:plan-phase {N} ${GSD_WS}`". Exit workflow.
   - "Continue without UI-SPEC" → Continue to step 6.
   - "Not a frontend phase" → Continue to step 6.
 
@@ -411,7 +411,7 @@ VALIDATION_EXISTS=$(ls "${PHASE_DIR}"/*-VALIDATION.md 2>/dev/null | head -1)
 ```
 
 If missing and Nyquist is still enabled/applicable — ask user:
-1. Re-run: `/gsd:plan-phase {PHASE} --research`
+1. Re-run: `/gsd:plan-phase {PHASE} --research ${GSD_WS}`
 2. Disable Nyquist with the exact command:
    `node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-set workflow.nyquist_validation false`
 3. Continue anyway (plans fail Dimension 8)
@@ -703,7 +703,7 @@ Plans ready. Launching execute-phase...
 
 Launch execute-phase using the Skill tool to avoid nested Task sessions (which cause runtime freezes due to deep agent nesting):
 ```
-Skill(skill="gsd:execute-phase", args="${PHASE} --auto --no-transition")
+Skill(skill="gsd:execute-phase", args="${PHASE} --auto --no-transition ${GSD_WS}")
 ```
 
 The `--no-transition` flag tells execute-phase to return status after verification instead of chaining further. This keeps the auto-advance chain flat — each phase runs at the same nesting level rather than spawning deeper Task agents.
@@ -717,14 +717,14 @@ The `--no-transition` flag tells execute-phase to return status after verificati
 
   Auto-advance pipeline finished.
 
-  Next: /gsd:discuss-phase ${NEXT_PHASE} --auto
+  Next: /gsd:discuss-phase ${NEXT_PHASE} --auto ${GSD_WS}
   ```
 - **GAPS FOUND / VERIFICATION FAILED** → Display result, stop chain:
   ```
   Auto-advance stopped: Execution needs review.
 
   Review the output above and continue manually:
-  /gsd:execute-phase ${PHASE}
+  /gsd:execute-phase ${PHASE} ${GSD_WS}
   ```
 
 **If neither `--auto` nor config enabled:**
@@ -755,7 +755,7 @@ Verification: {Passed | Passed with override | Skipped}
 
 **Execute Phase {X}** — run all {N} plans
 
-/gsd:execute-phase {X}
+/gsd:execute-phase {X} ${GSD_WS}
 
 <sub>/clear first → fresh context window</sub>