feat: Add configurable polecat branch naming (#825)

feat: Add configurable polecat branch naming

Adds polecat_branch_template configuration for custom branch naming patterns.

Template variables supported:
- {user}: git config user.name  
- {year}/{month}: date (YY/MM format)
- {name}: polecat name
- {issue}: issue ID without prefix
- {description}: sanitized issue title
- {timestamp}: unique timestamp

Maintains backward compatibility - empty template uses existing format.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **Configurable polecat branch naming** - Rigs can now customize polecat branch naming via `polecat_branch_template` configuration. Supports template variables: `{user}`, `{year}`, `{month}`, `{name}`, `{issue}`, `{description}`, `{timestamp}`. Defaults to existing behavior for backward compatibility.
+
 ### Fixed
 
 - **Orphan cleanup skips valid tmux sessions** - `gt orphans kill` and automatic orphan cleanup now check for Claude processes belonging to valid Gas Town tmux sessions (gt-*/hq-*) before killing. This prevents false kills of witnesses, refineries, and deacon during startup when they may temporarily show TTY "?"