fix(desktop): defer agent page secondary requests

[wesbillman]

Summary

• defer the expensive get_channels query on the Agents page until after the initial view commits
• keep channel-aware stop/delete flows safe by fetching channels on demand if the background query has not completed
• avoid mounting closed Agents-page dialogs so runtime/provider discovery and prereq checks do not run until the dialog opens
• avoid mounting each row's edit dialog until the user opens it

Validation

• bin/pnpm --dir desktop typecheck
• bin/pnpm --dir desktop check
• bin/pnpm --dir desktop build (existing Vite chunk-size / ineffective-dynamic-import warnings only)
• git diff --check
• pre-commit hook via git commit (desktop/web/mobile/rust format/check hooks)
• pre-push hook via CMAKE_POLICY_VERSION_MINIMUM=3.5 git push -u origin pinky/async-agent-page (desktop-test, rust-tests, mobile-test, desktop-tauri-test)

Note: the first pre-push attempt failed before running app tests because the local CMake version rejected the vendored Opus project's old cmake_minimum_required; setting CMAKE_POLICY_VERSION_MINIMUM=3.5 allowed the existing dependency build to proceed, and the hook then passed.