chore: migrate from opencode to trellis 0.5.0-rc.6

- Remove legacy .opencode/ directory and configuration
- Update .trellis/ to v0.5.0-rc.6 structure
- Refactor scripts: modularize common/, remove multi_agent/
- Add new common modules: git.py, io.py, log.py, types.py, etc.
- Update workflow.md and AGENTS.md
- Archive completed migration tasks

.trellis/scripts/common/log.py

@@ -0,0 +1,45 @@
+"""
+Terminal output utilities: colors and structured logging.
+
+Single source of truth for Colors and log_* functions
+used across all Trellis scripts.
+"""
+
+from __future__ import annotations
+
+
+class Colors:
+    """ANSI color codes for terminal output."""
+
+    RED = "\033[0;31m"
+    GREEN = "\033[0;32m"
+    YELLOW = "\033[1;33m"
+    BLUE = "\033[0;34m"
+    CYAN = "\033[0;36m"
+    DIM = "\033[2m"
+    NC = "\033[0m"  # No Color / Reset
+
+
+def colored(text: str, color: str) -> str:
+    """Apply ANSI color to text."""
+    return f"{color}{text}{Colors.NC}"
+
+
+def log_info(msg: str) -> None:
+    """Print info-level message with [INFO] prefix."""
+    print(f"{Colors.BLUE}[INFO]{Colors.NC} {msg}")
+
+
+def log_success(msg: str) -> None:
+    """Print success message with [SUCCESS] prefix."""
+    print(f"{Colors.GREEN}[SUCCESS]{Colors.NC} {msg}")
+
+
+def log_warn(msg: str) -> None:
+    """Print warning message with [WARN] prefix."""
+    print(f"{Colors.YELLOW}[WARN]{Colors.NC} {msg}")
+
+
+def log_error(msg: str) -> None:
+    """Print error message with [ERROR] prefix."""
+    print(f"{Colors.RED}[ERROR]{Colors.NC} {msg}")