Add runtime configuration for LLM commands (gt-dc2fs)

Add RuntimeConfig type to RigSettings allowing per-rig LLM runtime
configuration. This moves hardcoded "claude --dangerously-skip-permissions"
invocations to configurable settings.

Changes:
- Add RuntimeConfig type with Command, Args, InitialPrompt fields
- Add BuildCommand() and BuildCommandWithPrompt() methods
- Add helper functions: LoadRuntimeConfig, BuildAgentStartupCommand,
  BuildPolecatStartupCommand, BuildCrewStartupCommand
- Update startup paths in up.go and mayor.go to use new config
- Add comprehensive tests for RuntimeConfig functionality

Remaining hardcoded invocations can be updated incrementally.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/cmd/mayor.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/config"
 	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/style"
@@ -133,7 +134,8 @@ func startMayorSession(t *tmux.Tmux) error {
 	// Launch Claude - the startup hook handles 'gt prime' automatically
 	// Use SendKeysDelayed to allow shell initialization after NewSession
 	// Export GT_ROLE and BD_ACTOR in the command since tmux SetEnvironment only affects new panes
-	claudeCmd := `export GT_ROLE=mayor BD_ACTOR=mayor GIT_AUTHOR_NAME=mayor && claude --dangerously-skip-permissions`
+	// Mayor uses default runtime config (empty rigPath) since it's not rig-specific
+	claudeCmd := config.BuildAgentStartupCommand("mayor", "mayor", "", "")
 	if err := t.SendKeysDelayed(MayorSessionName, claudeCmd, 200); err != nil {
 		return fmt.Errorf("sending command: %w", err)
 	}