fix: filter empty error messages to prevent session corruption

When 429/500 errors occur during tool execution, empty assistant messages
with stopReason='error' and content=[] get persisted to the session file.

These break the tool_use -> tool_result chain that Claude/Gemini require:
- Claude expects every tool_use block to have a matching tool_result
- Empty error messages inserted mid-sequence violate this invariant
- Results in: 'tool_use ids were found without tool_result blocks'

This patch filters out empty error messages when building session context,
allowing sessions to recover gracefully from transient API errors.

Evidence from production:
- 113 of 170 sessions had empty error messages
- Session 30764430 demonstrated recovery: 429 at 14:30:11 IST,
  resumed successfully at 14:30:22, completed at 14:30:34

Sorry Peter, one more patch! 🙈

patches/@mariozechner__pi-coding-agent.patch

@@ -26,3 +26,23 @@ index 0000000..1111111 100644
  }
  /** Result from createAgentSession */
  export interface CreateAgentSessionResult {
+diff --git a/dist/core/session-manager.js b/dist/core/session-manager.js
+index b2aba5280d002253b0938b75aedbb9e6e6c4dcf8..67464efff535dbd7a8e6ed825aab2b305ca2aee2 100644
+--- a/dist/core/session-manager.js
++++ b/dist/core/session-manager.js
+@@ -161,6 +161,15 @@ export function buildSessionContext(entries, leafId, byId) {
+     const messages = [];
+     const appendMessage = (entry) => {
+         if (entry.type === "message") {
++            // PATCH: Filter out empty error assistant messages to prevent session corruption
++            // When 429/500 errors occur during tool execution, empty error messages get persisted
++            // to the session file. These break the tool_use -> tool_result chain for Claude/Gemini.
++            const msg = entry.message;
++            if (msg.role === "assistant" &&
++                msg.stopReason === "error" &&
++                (!msg.content || msg.content.length === 0)) {
++                return; // Skip empty error messages
++            }
+             messages.push(entry.message);
+         }
+         else if (entry.type === "custom_message") {