fix: persist Connection.aiRules through StoredConnection and CloudKit sync

[datlechin]

Summary

PR #1081 (per-connection AI rules) added aiRules: String? to DatabaseConnection.Codable, but missed two downstream layers:

1. StoredConnection (private struct in ConnectionStorage.swift) is the on-disk JSON format — separate from DatabaseConnection.Codable. Without aiRules here, the field is silently dropped on every saveConnections / loadConnections round-trip. User-visible bug: set rules in the AI Rules pane, save, relaunch the app — rules are gone.

2. SyncRecordMapper (Core/Sync/SyncRecordMapper.swift) maps DatabaseConnection to CKRecord for iCloud sync. Without aiRules here, the field never propagates between devices.

This PR adds aiRules to both layers, mirroring the established pattern (e.g. aiPolicy, aiAlwaysAllowedTools from PR E).

Test plan

• Edit a connection, set custom AI Rules text, click Save.
• Quit and relaunch the app. Open the connection's AI Rules pane — text persists.
• On a second iCloud-synced device: connection's AI Rules text matches.
• Verify the rules are still injected into the chat system prompt for that connection (existing PR G behavior — no regression).

[chatgpt-codex-connector]

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