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/manager.md

@@ -20,6 +20,7 @@ Bootstrap via manager init:
 
 ```bash
 INIT=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" init manager)
+if [[ "$INIT" == @file:* ]]; then INIT=$(cat "${INIT#@file:}"); fi
 ```
 
 Parse JSON for: `milestone_version`, `milestone_name`, `phase_count`, `completed_count`, `in_progress_count`, `phases`, `recommended_actions`, `all_complete`, `waiting_signal`.
@@ -53,6 +54,7 @@ Proceed to dashboard step.
 
 ```bash
 INIT=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" init manager)
+if [[ "$INIT" == @file:* ]]; then INIT=$(cat "${INIT#@file:}"); fi
 ```
 
 Parse the full JSON. Build the dashboard display.