feat(editor): nest generated _gsx.go files under .gsx sources in VS Code (#109)

* docs: document v0.18.0 API additions

Cover the new public API shipping in v0.18.0 across the reference docs,
llm.md, and the CLAUDE.md option pointers:

- Border title alignment (WithBorderTitleAlign), independent title and
  focus border styling (WithBorderTitleStyle, WithFocusBorderStyle)
- App render hooks (WithPreRenderHook, WithPostRenderHook) and
  WithManualCursor
- Real-cursor protocol (WithCursorSource/SetCursorSource, CursorReporter)
  and the per-widget virtual-cursor opt-in
- TextArea/Input programmatic editing (CursorPos, SetCursorPos, InsertText)
  and WithTextAreaOnChange
- Element runtime resize setters (SetWidth, SetHeight)

* feat(editor): nest generated _gsx.go files under .gsx sources in VS Code

The VS Code extension now contributes an explorer.fileNesting.patterns
default (*.gsx -> ${capture}_gsx.go) so each generated file collapses
under its source in the Explorer. The pattern ships disabled-by-default
(VS Code's fileNesting master switch stays off), so installing the
extension never reorganizes unrelated repositories.

A repo-level .vscode/settings.json enables nesting for go-tui itself and
repeats the pattern so it works without the extension installed; .gitignore
is narrowed to track that one shared settings file. README and CLI docs
document the one-line opt-in and the hyphenated-filename caveat.