fix(desktop): apply Claude models and run Windows installers natively

[tagadearpit]

Summary

• inject ANTHROPIC_MODEL from the structured Claude Code model picker at spawn time
• treat ANTHROPIC_MODEL as derived configuration so stale custom environment values cannot override later picker changes
• run catalogued Windows PowerShell installers directly instead of routing them through Git Bash, preventing MSYS path conversion from corrupting native installer paths
• add regression coverage for model injection and native PowerShell command construction

Root cause

Claude Code's runtime catalog had no model environment variable, so the picker value was saved but never reached the spawned Claude process. On Windows, Claude and Codex installer scripts were invoked through Git Bash; MSYS path conversion could turn C: paths into invalid arguments for native tools such as tar.

Closes #2692
Closes #2688

Issue #2690 is intentionally not duplicated here: upstream PR #2694 already addresses the macOS artifact-name collision.

Validation

• cargo fmt --all -- --check
• git diff --check
• verified upstream comparison: one commit ahead of block/buzz:main, with only the five intended files
• targeted Rust tests could not link in this Linux workspace because pkg-config and the required GLib/GObject development libraries are unavailable; CI should run the full platform matrix.