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/quick.md

@@ -691,7 +691,7 @@ Commit: ${commit_hash}
 
 ---
 
-Ready for next task: /gsd:quick
+Ready for next task: /gsd:quick ${GSD_WS}
 ```
 
 **If NOT `$FULL_MODE`:**
@@ -708,7 +708,7 @@ Commit: ${commit_hash}
 
 ---
 
-Ready for next task: /gsd:quick
+Ready for next task: /gsd:quick ${GSD_WS}
 ```
 
 </process>