docs: fix a2a skill setup and binary naming

[Math1987]

Description

The skill's documented go install command creates an executable named a2a-cli, while its examples and verification step invoke a2a. Build explicitly with go build -o a2a . and explain the PATH setup. Replace the skill's automatic installation instruction with an a2a version preflight and a link to the setup instructions.

This replaces #43, which GitHub could not reopen after its original base branch (skill/a2a-cli) was deleted. Per maintainer request, this PR targets main. It implements the previously posted patch. Related issue: #4.

Validation

• Reproduced the exact latest-version installation in an isolated GOBIN: it creates a2a-cli. Latest resolved to v0.0.0-20260904130516-a27e1ee6d332.
• Verified the explicit build, PATH setup and a2a version against PR docs: add a2a-cli agent skill #27's head on macOS arm64 with Go 1.26.0.
• git diff --check passes.

Which area does this relate to?

• CLI code
• SPEC or COMPLIANCE file
• Other doc files

Checklist

• Reviewed the contributing guide.
• Conventional Commits title.
• Appropriate docs updated.

Go formatting, unit tests and lint are not applicable to this documentation-only change.

Maintainer action

The linked-issue check requires a GitHub closing-issue association, and my account cannot add that association. Please link this PR to the existing issue #4 using the repository's preferred process.

[msampathkumar]

@Math1987 - Thank you for the PR.

[yarolegovich]

thanks for offering to help with that. it's unfortunate go install does not allow a simple rename.
closing this because it will be addressed in #47