Add file conflict handling to minio-upload skill

Implement comprehensive file conflict detection and resolution:
- Check for existing files before upload using mc stat (fixes mc ls bug)
- Interactive prompt when conflict detected: overwrite, skip, backup, rename
- Command-line options for non-interactive use: --force, --skip, --backup, --rename
- Auto-rename with numeric suffix when file exists
- Backup existing files with .bak extension before overwriting
- Update SKILL.md with conflict handling documentation and examples

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

skills/minio-upload/SKILL.md

@@ -19,6 +19,47 @@ Upload to a specific vault:
 .claude/skills/minio-upload/scripts/upload_to_obsidian.sh path/to/file.md "journal"
 ```
 
+## File Conflict Handling
+
+**CRITICAL**: When a file with the same name already exists in the target vault, the script will prompt you to choose an action (unless an option is specified).
+
+### Interactive Prompt (Default)
+
+If the file exists and no option is provided, you'll see:
+```
+⚠️  File 'filename.md' already exists in vault 'main'
+Choose an action:
+  [o] Overwrite - Replace existing file
+  [s] Skip      - Cancel upload
+  [b] Backup    - Backup existing file (.bak) then overwrite
+  [r] Rename    - Upload with new name (adds numeric suffix)
+```
+
+### Command-Line Options
+
+You can specify the behavior explicitly to skip the prompt:
+
+- `--force` - Overwrite existing files without prompting
+- `--skip` - Skip upload if file already exists
+- `--backup` - Backup existing file (creates `.bak` file) before overwriting
+- `--rename` - Automatically rename with numeric suffix (e.g., `file-1.md`, `file-2.md`)
+
+### Examples
+
+```bash
+# Force overwrite without prompting
+upload_to_obsidian.sh file.md main --force
+
+# Skip if file exists
+upload_to_obsidian.sh file.md journal --skip
+
+# Backup before overwriting
+upload_to_obsidian.sh file.md work --backup
+
+# Auto-rename if conflict
+upload_to_obsidian.sh file.md personal --rename
+```
+
 ## Vault Configuration
 
 **CRITICAL RULE**: When no vault is explicitly specified by the user, files MUST be uploaded to the `main` vault only. Do NOT assume or invent other vault names.
@@ -52,8 +93,10 @@ The MinIO obsidian bucket contains Obsidian vaults as top-level directories:
 
 1. **Verify file exists** - Check that the source file is accessible
 2. **Determine target vault** - Use specified vault name or default to `main`
-3. **Upload with mc** - Execute `mc cp` command to transfer file
-4. **Confirm success** - Verify upload completed and report target location
+3. **Check for conflicts** - Detect if file already exists in target vault
+4. **Resolve conflict** - Prompt user or use specified option to handle conflict
+5. **Upload with mc** - Execute `mc cp` command to transfer file
+6. **Confirm success** - Verify upload completed and report target location
 
 ## Error Handling