feat: per-connection AI rules

[datlechin]

Summary

PR G of the chat-roadmap-cdfg track. Adds a per-connection AI Rules field so users can pin connection-specific guidance (table conventions, PII columns, naming, business rules) that the assistant sees on every chat turn for that connection.

• New aiRules: String? on DatabaseConnection. Codable via decodeIfPresent for forward compatibility, encoded with encodeIfPresent. Round-trips through the existing keychain-paired UserDefaults storage.
• New AI Rules pane in the connection form sidebar (next to General / SSH / SSL / Customization / Advanced). Multiline editor with placeholder examples for what to write.
• AISchemaContext.buildSystemPrompt accepts an optional connectionRules; when non-empty, appends a ## Connection-Specific Rules section to the system prompt. AIChatViewModel.PromptContext carries the rules through.

Out of scope: schema-aware rule suggestions, .tableprorules files on disk, AI-generated rule suggestions, per-table or per-column rules, sync/export.

Test plan

• Add a connection, open AI Rules in the form sidebar, type rules, save. Reopen the connection: rules persist.
• Open AI Chat against that connection. Confirm the system prompt includes a ## Connection-Specific Rules section with the rules text (verifiable by inspecting the prompt or via the included tests).
• On a connection without rules, the system prompt has no rules section.
• xcodebuild -project TablePro.xcodeproj -scheme TablePro test -skipPackagePluginValidation -only-testing:TableProTests/DatabaseConnectionAIRulesTests passes.
• swiftlint lint --strict passes for changed files.

[chatgpt-codex-connector]

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