Fix deacon patrol process leak by killing pane processes before respawn (#745)

## Problem
The deacon patrol was leaking claude processes. Every patrol cycle (1-3 minutes),
a new claude process was spawned under the hq-deacon tmux session, but old processes
were never terminated. This resulted in 12+ accumulated claude processes consuming
resources.

## Root Cause
In molecule_step.go:331, handleStepContinue() used tmux respawn-pane -k to restart
the pane between patrol steps. The -k flag sends SIGHUP to the shell but does not
kill all descendant processes (claude and its node children).

## Solution
Added KillPaneProcesses() function in tmux.go that explicitly kills all descendant
processes before respawning the pane. This function:
- Gets all descendant PIDs recursively
- Sends SIGTERM to all (deepest first)
- Waits 100ms for graceful shutdown
- Sends SIGKILL to survivors

Updated handleStepContinue() to call KillPaneProcesses() before RespawnPane().

Co-authored-by: Roland Tritsch <roland@ailtir.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/cmd/molecule_step.go

@@ -322,6 +322,12 @@ func handleStepContinue(cwd, townRoot, _ string, nextStep *beads.Issue, dryRun b
 
 	t := tmux.NewTmux()
 
+	// Kill all processes in the pane before respawning to prevent process leaks
+	if err := t.KillPaneProcesses(pane); err != nil {
+		// Non-fatal but log the warning
+		style.PrintWarning("could not kill pane processes: %v", err)
+	}
+
 	// Clear history before respawn
 	if err := t.ClearHistory(pane); err != nil {
 		// Non-fatal