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.

docs/reference.md

@@ -89,6 +89,58 @@ Debug routing: `BD_DEBUG_ROUTING=1 bd show <id>`
 
 Process state, PIDs, ephemeral data.
 
+### Rig-Level Configuration
+
+Rigs support layered configuration through:
+1. **Wisp layer** (`.beads-wisp/config/`) - transient, local overrides
+2. **Rig identity bead labels** - persistent rig settings
+3. **Town defaults** (`~/gt/settings/config.json`)
+4. **System defaults** - compiled-in fallbacks
+
+#### Polecat Branch Naming
+
+Configure custom branch name templates for polecats:
+
+```bash
+# Set via wisp (transient - for testing)
+echo '{"polecat_branch_template": "adam/{year}/{month}/{description}"}' > \
+  ~/gt/.beads-wisp/config/myrig.json
+
+# Or set via rig identity bead labels (persistent)
+bd update gt-rig-myrig --labels="polecat_branch_template:adam/{year}/{month}/{description}"
+```
+
+**Template Variables:**
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `{user}` | From `git config user.name` | `adam` |
+| `{year}` | Current year (YY format) | `26` |
+| `{month}` | Current month (MM format) | `01` |
+| `{name}` | Polecat name | `alpha` |
+| `{issue}` | Issue ID without prefix | `123` (from `gt-123`) |
+| `{description}` | Sanitized issue title | `fix-auth-bug` |
+| `{timestamp}` | Unique timestamp | `1ks7f9a` |
+
+**Default Behavior (backward compatible):**
+
+When `polecat_branch_template` is empty or not set:
+- With issue: `polecat/{name}/{issue}@{timestamp}`
+- Without issue: `polecat/{name}-{timestamp}`
+
+**Example Configurations:**
+
+```bash
+# GitHub enterprise format
+"adam/{year}/{month}/{description}"
+
+# Simple feature branches
+"feature/{issue}"
+
+# Include polecat name for clarity
+"work/{name}/{issue}"
+```
+
 ## Formula Format
 
 ```toml