fix: use cd + worktree inside parent dir instead of --dir flag

Issue #105: opencode run --fork/--continue --dir <path> fails to create sessions

Root cause: The --dir flag breaks session creation in opencode. Sessions
fail to be created when --dir is used with --fork or --continue.

Solution: Instead of using --dir flag, create worktrees inside the parent
session's directory and use 'cd $worktree_path && opencode run ...' to
change directory before running opencode.

Key changes:
- Worktrees now created at $PWD/.kugetsu-worktrees/{issue-ref}/ instead
  of $WORKTREES_DIR/{issue-ref}/
- .kugetsu-worktrees is a hidden directory (git ignored by default)
- cmd_start and cmd_continue now use 'cd && opencode run' instead of
  'opencode run --dir'

This approach works because:
1. Worktree is inside parent's directory tree (permission granted)
2. cd properly changes working directory before opencode runs
3. Session gets created with correct directory set
4. No .gitignore entry needed (. prefix makes it hidden from git)

skills/kugetsu/scripts/kugetsu

@@ -146,8 +146,9 @@ issue_ref_to_worktree_name() {
 
 issue_ref_to_worktree_path() {
     local issue_ref="$1"
+    local parent_dir="${2:-$WORKTREES_DIR}"
     local worktree_name=$(issue_ref_to_worktree_name "$issue_ref")
-    echo "$WORKTREES_DIR/$worktree_name"
+    echo "$parent_dir/.kugetsu-worktrees/$worktree_name"
 }
 
 issue_ref_to_branch_name() {
@@ -195,13 +196,15 @@ get_repo_url() {
 
 worktree_exists() {
     local issue_ref="$1"
-    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref")
+    local parent_dir="${2:-$PWD}"
+    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref" "$parent_dir")
     [ -d "$worktree_path" ]
 }
 
 create_worktree() {
     local issue_ref="$1"
-    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref")
+    local parent_dir="${2:-$PWD}"
+    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref" "$parent_dir")
     local branch_name=$(issue_ref_to_branch_name "$issue_ref")
     local repo_url=$(get_repo_url "$issue_ref")
     
@@ -211,9 +214,10 @@ create_worktree() {
         exit 1
     fi
     
-    ensure_worktree_dir
+    local worktree_parent_dir=$(dirname "$worktree_path")
+    mkdir -p "$worktree_parent_dir"
     
-    if worktree_exists "$issue_ref"; then
+    if worktree_exists "$issue_ref" "$parent_dir"; then
         echo "Removing existing worktree at '$worktree_path'..."
         git worktree remove "$worktree_path" 2>/dev/null || rm -rf "$worktree_path"
     fi
@@ -234,9 +238,10 @@ create_worktree() {
 
 remove_worktree_for_issue() {
     local issue_ref="$1"
-    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref")
+    local parent_dir="${2:-$PWD}"
+    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref" "$parent_dir")
     
-    if worktree_exists "$issue_ref"; then
+    if worktree_exists "$issue_ref" "$parent_dir"; then
         echo "Removing worktree at '$worktree_path'..."
         git worktree remove "$worktree_path" 2>/dev/null || rm -rf "$worktree_path"
     fi
@@ -1215,8 +1220,9 @@ cmd_start() {
         exit 1
     fi
 
-    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref")
-    create_worktree "$issue_ref"
+    local parent_dir="$PWD"
+    local worktree_path=$(issue_ref_to_worktree_path "$issue_ref" "$parent_dir")
+    create_worktree "$issue_ref" "$parent_dir"
 
     local session_file="$(issue_ref_to_filename "$issue_ref").json"
 
@@ -1233,14 +1239,14 @@ cmd_start() {
     if [ "$active_count" -ge "$MAX_CONCURRENT_AGENTS" ]; then
         echo "Error: Max concurrent agents ($MAX_CONCURRENT_AGENTS) reached" >&2
         echo "Active sessions: $active_count" >&2
-        remove_worktree_for_issue "$issue_ref"
+        remove_worktree_for_issue "$issue_ref" "$parent_dir"
         exit 1
     fi
     
     if [ "$DEBUG_MODE" = true ]; then
-        opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1 | tee "$SESSIONS_DIR/$session_file.debug.log" &
+        (cd "$worktree_path" && opencode run "$message" --fork --session "$base_session_id" 2>&1) | tee "$SESSIONS_DIR/$session_file.debug.log" &
     else
-        opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1 &
+        (cd "$worktree_path" && opencode run "$message" --fork --session "$base_session_id" 2>&1) &
     fi
 
     # Wait briefly for session to be created
@@ -1331,9 +1337,9 @@ cmd_continue() {
     if [ -n "$worktree_path" ] && [ -d "$worktree_path" ]; then
         echo "Using worktree: $worktree_path"
         if [ "$DEBUG_MODE" = true ]; then
-            opencode run "$message" --continue --session "$opencode_session_id" --dir "$worktree_path" 2>&1 | tee "$session_path.debug.log" &
+            (cd "$worktree_path" && opencode run "$message" --continue --session "$opencode_session_id" 2>&1) | tee "$session_path.debug.log" &
         else
-            opencode run "$message" --continue --session "$opencode_session_id" --dir "$worktree_path" 2>&1 &
+            (cd "$worktree_path" && opencode run "$message" --continue --session "$opencode_session_id" 2>&1) &
         fi
     else
         if [ "$DEBUG_MODE" = true ]; then