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/plan-phase.md

@@ -339,7 +339,7 @@ If `research_enabled` is false and `nyquist_validation_enabled` is true: warn "N
 In that case: **skip validation-strategy creation entirely**. Do **not** expect `RESEARCH.md` or `VALIDATION.md` for this run, and continue to Step 6.
 
 ```bash
-grep -l "## Validation Architecture" "${PHASE_DIR}"/*-RESEARCH.md 2>/dev/null
+grep -l "## Validation Architecture" "${PHASE_DIR}"/*-RESEARCH.md 2>/dev/null || true
 ```
 
 **If found:**
@@ -409,7 +409,7 @@ Otherwise use AskUserQuestion:
 ## 6. Check Existing Plans
 
 ```bash
-ls "${PHASE_DIR}"/*-PLAN.md 2>/dev/null
+ls "${PHASE_DIR}"/*-PLAN.md 2>/dev/null || true
 ```
 
 **If exists AND `--reviews` flag:** Skip prompt — go straight to replanning (the purpose of `--reviews` is to replan with review feedback).