chore: make fmt a format check, update go-ci.yml to @v2

[cboone]

Summary

• Change make fmt from write mode (go fmt ./...) to a format check that exits non-zero when files need formatting
• Add make format target for write-mode formatting
• Update go-ci.yml reference from @v1 to @v2

go-ci.yml v2 runs make fmt as a format check. The convention is that fmt checks and format writes.

Test plan

• CI passes with go-ci.yml@v2
• make fmt exits 0 on clean code, exits 1 after breaking formatting
• make format still formats code

[Copilot]

Pull request overview

This PR changes make fmt from a write-mode formatter (go fmt ./...) to a format check that exits non-zero when files need formatting, adds a new make format target for write-mode formatting, and updates the CI workflow to use go-ci.yml@v2 which relies on this check convention.

Changes:

• make fmt now runs gofmt -l . to detect unformatted files and exits 1 if any are found
• New make format target added to perform actual formatting via go fmt ./...
• CI workflow updated from go-ci.yml@v1 to go-ci.yml@v2

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Makefile	Split fmt into a check-only target and a new format write target; updated help text
.github/workflows/ci.yml	Updated reusable workflow reference from @v1 to @v2

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[cboone]

Copilot Feedback Resolved

Addressed 2 Copilot review comment(s) with code changes:

File	Category	Action
Makefile:39	Valid	Removed redundant ## inline comments from fmt and format
Makefile:35	Incorrect	Cross-repo convention; added Copilot instruction