fix: Windows shell robustness, project_root detection, and hook stdin safety (#1343)

Address 4 root causes of Windows + Claude Code reliability issues:

1. Workflow shell robustness: add || true guards to informational commands
   (ls, grep, find, cat) that return non-zero on "no results", preventing
   workflow step failures under strict execution models. Guard glob loops
   with [ -e "$var" ] || continue to handle empty glob expansion.

2. Hook stdin handling: replace readFileSync('/dev/stdin') with async
   process.stdin + timeout in agent templates (gsd-verifier.md). Existing
   JS hooks already have timeout guards.

3. project_root detection: fix isInsideGitRepo() to check .git at the
   candidate parent level (not just below it), enabling correct detection
   when .git and .planning/ are siblings at the same directory level —
   the common single-repo case from a subdirectory.

4. @file: handoff: add missing @file: handlers to autonomous.md and
   manager.md workflows that call gsd-tools init but lacked the handler
   for large output payloads.

Fixes #1343

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

get-shit-done/workflows/verify-work.md

@@ -35,7 +35,7 @@ Parse JSON for: `planner_model`, `checker_model`, `commit_docs`, `phase_found`,
 **First: Check for active UAT sessions**
 
 ```bash
-find .planning/phases -name "*-UAT.md" -type f 2>/dev/null | head -5
+(find .planning/phases -name "*-UAT.md" -type f 2>/dev/null || true) | head -5
 ```
 
 **If active sessions exist AND no $ARGUMENTS provided:**
@@ -84,7 +84,7 @@ Continue to `create_uat_file`.
 Use `phase_dir` from init (or run init if not already done).
 
 ```bash
-ls "$phase_dir"/*-SUMMARY.md 2>/dev/null
+ls "$phase_dir"/*-SUMMARY.md 2>/dev/null || true
 ```
 
 Read each SUMMARY.md to extract testable deliverables.