fix: add Windows Git Bash fallbacks for uname, date, and cat

[sampurnamo]

Summary

In restricted shell contexts on Windows (Git Bash running within hook processes), core POSIX utilities like uname, date, and cat may not be available in $PATH. This causes the mira-wrapper to fail with errors like:

line 47: date: command not found

This PR guards all shell utility calls with 2>/dev/null fallbacks:

• uname -s: Falls back to "Windows" — safe default that triggers Windows code paths
• uname -m: Falls back to "x86_64" — most common Windows architecture
• date +%s: Falls back to the file mtime being compared — produces cache_age=0, which means "cache is fresh" (safe: skips unnecessary update checks)
• cat: Falls back to "" — triggers re-download path (safe)
• Windows* pattern added to all case statements that lacked it

Test plan

• Verify wrapper still works on Linux/macOS (uname available — fallbacks never trigger)
• Verify wrapper works in Windows Git Bash with full PATH
• Verify wrapper works in restricted Windows hook contexts (stripped PATH)
• Confirm install_jq() handles Windows detection correctly

Replaces #10 (which had merge conflicts from fork divergence).

[TusanHomichi]

Code review

Found 1 issue:

1. date +%s fallback produces zero age, defeating both the cache TTL check and stale lock cleanup. When date +%s fails, the fallback substitutes the mtime value itself (echo "$file_mtime" / echo "$lock_mtime"), making the arithmetic (mtime - mtime) = 0. This means:

   • cache_age=0 always passes the freshness check (0 < 86400), so update checks are permanently skipped
   • lock_age=0 never satisfies lock_age > 60, so stale locks are never reclaimed (potential deadlock after crash)

   The correct fallback is echo 0 (Unix epoch), which produces a very large age and forces the safe path (re-check updates / clean up stale locks):

   cache_age=$(( $(date +%s 2>/dev/null || echo 0) - file_mtime ))
   local lock_age=$(( $(date +%s 2>/dev/null || echo 0) - lock_mtime ))

   Mira/plugin/bin/mira-wrapper

   Lines 68 to 74 in 275c03a

   	# Accept any installed version if update cache is fresh
   	if [ -f "$UPDATE_CACHE" ]; then
   	file_mtime=$(stat -c %Y "$UPDATE_CACHE" 2>/dev/null || stat -f %m "$UPDATE_CACHE" 2>/dev/null || date -r "$UPDATE_CACHE" +%s 2>/dev/null || echo 0)
   	cache_age=$(( $(date +%s 2>/dev/null || echo "$file_mtime") - file_mtime ))
   	if [ "$cache_age" -lt "$CACHE_TTL" ]; then
   	exec "$MIRA_BIN" "$@"
   	fi

   Mira/plugin/bin/mira-wrapper

   Lines 383 to 391 in 275c03a

   	if ! mkdir "$lockdir" 2>/dev/null; then
   	local lock_mtime
   	lock_mtime=$(stat -c %Y "$lockdir" 2>/dev/null || stat -f %m "$lockdir" 2>/dev/null || echo 0)
   	local lock_age=$(( $(date +%s 2>/dev/null || echo "$lock_mtime") - lock_mtime ))
   	if [ "$lock_age" -gt 60 ]; then
   	rmdir "$lockdir" 2>/dev/null || true
   	mkdir "$lockdir" 2>/dev/null || return
   	else
   	return # Another process is writing -- skip

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