Release v0.4.1: support desktop agent installation

[Azakeen]

Summary

• add a standalone Claude Desktop MCP installer with platform-aware configuration paths
• let Codex Desktop install, report status, migrate owned entries, and uninstall without requiring the Codex CLI binary
• add desktop aliases and detection for Codex and Gemini Code Assist for VS Code
• bump workspace and MCP metadata to 0.4.1
• update public documentation and remove internal benchmark attribution from public Markdown

Why

The installer treated provider support as equivalent to CLI availability. Desktop clients either use a separate configuration file or share configuration with their CLI, so desktop-only installations could be missed or fail during setup.

User impact

spectra install now detects supported desktop surfaces, and users can explicitly select claude-desktop, codex-desktop, or gemini-desktop. Existing settings remain ownership-protected and unrelated configuration is preserved.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• GitHub Actions CI passed on Ubuntu, macOS, and Windows
• signed release and package-manifest publication workflow passed

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