fix: Handle architecture-specific license dependencies

[SamMorrowDrums]

Summary

This PR improves the license script to handle architecture-specific dependency differences. Some dependencies have different module graphs for different GOARCH values (e.g., x/sys/cpu vs x/sys/unix), which can result in different licenses being exported per architecture.

Problem

As discovered in github/cli, some deps change which changes the mod graph for different GOARCH values. For example:

• amd64 might have x/sys/cpu and x/sys/unix so the common ancestor is x/sys
• arm64 might just have x/sys/unix so the common ancestor is x/sys/unix

This means license reports can differ between architectures on the same platform.

Solution

Updated script/licenses to:

1. Check all architectures per platform - linux (amd64, arm64, 386), darwin (amd64, arm64), windows (amd64, arm64, 386)
2. Group identical results - Architectures with the same dependencies are grouped together with comma-separated headers
3. Show divergent lists - Only shows separate sections when architectures actually differ
4. Add index at top - Each platform file now has a table of contents linking to each architecture section
5. Save per-arch licenses - Each architecture's licenses are saved in the third-party folder

Changes

• script/licenses: Enhanced to handle multi-architecture license generation with grouping
• third-party-licenses.*.md: Updated with architecture sections and index
• third-party/: Contains architecture-specific license folders

Example Output

When all architectures have the same deps:

## Index

- [amd64, arm64, 386](#amd64-arm64-386)

## amd64, arm64, 386

| Module | License |
|--------|---------|
| ... | ... |

When architectures differ:

## Index

- [amd64](#amd64)
- [arm64, 386](#arm64-386)

## amd64

| Module | License |
|--------|---------|
| ... | ... |

## arm64, 386

| Module | License |
|--------|---------|
| ... | ... |

[Copilot]

Pull request overview

This PR enhances the license generation script to properly handle architecture-specific dependency differences across GOOS/GOARCH combinations. The script now generates separate reports per architecture, groups identical results with comma-separated headers, and adds a table of contents to each platform file.

Key changes:

• Multi-architecture license generation for Linux (amd64, arm64, 386), Darwin (amd64, arm64), and Windows (amd64, arm64, 386)
• Grouping logic that consolidates architectures with identical dependency lists
• Automated table of contents generation with anchor links

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
script/licenses	Complete rewrite to support multi-architecture license generation with grouping logic, TOC generation, and per-arch report handling
third-party-licenses.linux.md	Restructured with TOC and architecture sections; packages reordered alphabetically
third-party-licenses.darwin.md	Restructured with TOC and architecture sections; packages reordered alphabetically
third-party-licenses.windows.md	Restructured with TOC and architecture sections; packages reordered alphabetically