feat: add read-only Gateway TUI

[aidandaly24]

Summary

• launch bare Gateway branches and read leaves into the TUI through the shared Handler tree while preserving flag-driven headless JSON behavior
• align TUI routes with the CLI command hierarchy for Gateway, Target, Connector, and Rule get/list
• use staged Gateway selection for nested resources, with direct bare get routes redirecting through the corresponding list flow
• present Connector as a separate resource experience while continuing to use Gateway Target APIs internally
• keep parent selection and scoped resource listing in dedicated Gateway, Target, Connector, and Rule picker components
• centralize Connector get/list filtering and validation in GatewayClient, with CLI and TUI consumers delegating through CoreGatewayClient
• fill Connector pages to maxResults by scanning Target pages with the remaining row count, preserving the final AWS token without custom pagination state
• use router-level supportedTuiCommands so read commands enter the TUI while bare CLI-only create commands run normal validation
• reuse the established router menu, paginated table, resource detail, JSON, loading, retry, empty, keyboard, and responsive layout components

Testing

• bun test src/handlers/gateway/gateway.test.tsx src/handlers/gateway/gateway.screen.test.tsx src/handlers/gateway/gateway.fixture.test.tsx src/core/gateway.test.ts src/components/ui/data-table/columnWidths.test.ts (86 pass)
• bun test --coverage --coverage-reporter=text (1061 pass; 93.77% functions, 97.85% lines)
• bun run typecheck
• bun run lint:check
• bun run format:check
• bun run build
• npm pack --ignore-scripts
• help smoke for Gateway root plus all eight get/list leaves (12/12)
• TUI harness at 100x30, confirming Gateway and Connector menus expose read-only commands only
• deploy-account pagination probe: reused a ListGatewayTargets token created with maxResults=2 in a request with maxResults=1, returning the next distinct Target

The full source run retains the two pre-existing local src/io/exec.test.ts child-output failures.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.32203% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.83%. Comparing base (74e9bd5) to head (c710a7a).

Files with missing lines	Patch %	Lines
src/handlers/gateway/get/screen.tsx	95.94%	3 Missing ⚠️
src/components/GatewayConnectorPicker.tsx	98.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #1956      +/-   ##
============================================
+ Coverage     96.74%   96.83%   +0.08%     
============================================
  Files           301      317      +16     
  Lines         16785    17355     +570     
============================================
+ Hits          16239    16806     +567     
- Misses          546      549       +3     

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