Add --layout filter flag to list-windows

[YuriNachos]

Closes #429.

Summary

Adds a --layout <layout> filter flag to list-windows, so the command prints only the windows whose effective layout matches — e.g. list-windows --layout floating to list only floating windows (the scratchpad use case from #429).

Problem

list-windows currently dumps every window across the target workspaces. #429 asks for a way to filter by the window's effective layout — floating windows in particular — which is useful for scripting and for inspecting how AeroSpace assigned windows to layouts.

Changes

• ListWindowsCmdArgs.swift — new --layout <target-layout> argument. It accepts the same layout tokens the layout command uses (floating, tiling, tiles, accordion, horizontal, vertical, h_tiles, v_tiles, h_accordion, v_accordion); an invalid token fails with Can't parse '--layout' argument. Possible values: ….
• format.swift — new windowMatchesLayout predicate. It derives the window's effective layout through the same getChildParentRelation source of truth that backs the %{window-layout} interpolation variable, so --layout floating keeps exactly what %{window-layout} prints as floating. The family tokens are inclusive: tiling matches every tiling variant, tiles/accordion match the layout family, horizontal/vertical match the orientation, and h_*/v_* match exactly. Windows with no parent or with an illegal/native/popup parent relation match nothing.
• ListWindowsCommand.swift — applies the filter before the count/format stage, so --count counts only the matching windows too.
• ListWindowsTest.swift — new testParseLayoutFilter (valid tokens parse; an invalid token is rejected with the documented error) and testRunFilterByLayout (one tiling and one floating window in a workspace; each filter returns only the matching window id). Behavior without the flag is unchanged.

Testing

• ./test.sh (build + test + lint + generate) — all tests passed.
• The new tests assert the filter returns only matching windows and that omitting the flag preserves the previous output.

Notes

Live end-to-end verification against real windows needs Accessibility permission and a running session; it is not exercised by the headless gate. The unit tests cover the filter logic; the build + lint + generate gate covers compilation.

[YuriNachos]

This adds the --layout filter to list-windows from #429, reusing the existing effective-layout derivation so the flag filters exactly what the command prints. Given the project's habit of shaping features in Discussions first, is the interface as implemented (regex argument named --layout) acceptable, or would you prefer to weigh the design in a Discussion before it merges? I'm happy to open one or adjust the flag shape either way.