feat: add read-only Gateway TUI

[aidandaly24]

Summary

• launch agentcore gateway into a read-only Gateway browser while preserving headless get, list, Target, Connector, and Rule commands
• browse Gateway summaries and JSON details, then navigate to scoped Target, Connector, and Rule lists and JSON details
• reuse the established paginated table, resource detail, JSON, loading, retry, empty, keyboard, and responsive layout components
• keep the TUI free of Create, Update, Delete, Invoke, and IAM actions
• correct the stale memoryColumns test import introduced by the latest refactor merge

Testing

• bun test src/handlers/gateway src/components/PaginatedTablePicker.test.tsx src/components/ui/data-table/columnWidths.test.ts (76 pass)
• bun test src/handlers/gateway --coverage --coverage-reporter=text (49 pass; all changed Gateway lines covered)
• bun run typecheck
• bun run lint:check
• bun run format:check
• bun run build
• TUI harness against the deploy profile at 120x40, 80x24, and 80x16, including Gateway filtering and selection, Gateway and Target JSON, Connector and Rule empty states, Escape navigation, responsive columns, and server pagination

The full source run completed with 977 passing tests and the two pre-existing local src/io/exec.test.ts child-output failures.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.84%. Comparing base (9cae679) to head (fbf99b8).

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #1956      +/-   ##
============================================
+ Coverage     95.70%   95.84%   +0.14%     
============================================
  Files           291      299       +8     
  Lines         16246    16616     +370     
============================================
+ Hits          15548    15926     +378     
+ Misses          698      690       -8     

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