Require pull requests for LLGo Binaryen changes

[cpunion]

Document the contribution path for xgo-dev/binaryen:llgo: changes come from a cpunion/binaryen branch through a pull request, and release tags follow review and integration.

The initial fork bootstrap was pushed before this policy was clarified. This PR establishes the process for subsequent changes; it does not rewrite the existing branch history.

Validation: git diff --check passed. This documentation change does not affect Binaryen binaries.

[fennoai]

Review summary

This PR adds a single documentation line to LLGO.md (line 7) codifying the contribution/release policy for the llgo branch: changes arrive via PR from cpunion/binaryen targeting xgo-dev/binaryen:llgo, no direct pushes to llgo, and release tags only after the contribution PR is reviewed and integrated.

This is a low-risk, documentation-only change. It reads clearly, matches the existing document style, and is consistent with the surrounding bullets (the release-branch role on line 6 and the tagging process on line 9). The policy is also security-positive: it establishes a trusted contribution path and prohibits direct pushes to the release branch.

No blocking issues. Two minor, optional observations:

• cpunion/binaryen namespace introduced without context (line 7): This is the first mention of the cpunion namespace in the file (alongside xgo-dev and WebAssembly). A brief clarifier such as "from a personal fork cpunion/binaryen" would make the fork relationship explicit for readers unfamiliar with the setup. Not blocking.
• Release-tag timing guidance now split across two bullets: The clause "Create a release tag only after the contribution PR has been reviewed and integrated" partially overlaps with line 9, which already covers tag creation (Tags named llgo-v132.N are created at the llgo tip). Consider consolidating the tag-timing guidance into the line 9 bullet to keep a single source of truth and avoid future drift. Not blocking.

Approving in spirit — these are optional polish suggestions.