chore: improve linting configuration and dev tooling

[min0625]

Summary

Refactors linting and dev tooling setup for better flexibility and correctness.

Changes

• .devcontainer/post_create.sh: Add go mod download before installing dev tools to ensure dependencies are available.
• .github/workflows/pr-check.yml: Pass NEW_FROM_REV=origin/${{ github.base_ref }} to make check so CI only reports lint issues introduced by the PR.
• .golangci.yaml: Remove new: true and new-from-rev: HEAD~ from config — incremental lint is now controlled via CLI flag for greater flexibility.
• Makefile:
  • Add NEW_FROM_REV ?= HEAD variable (defaults to HEAD for local use)
  • Apply --new-from-rev=$(NEW_FROM_REV) to lint and fix targets
  • Add go mod tidy to fix target
  • Add go mod tidy -diff to check target to verify go.mod/go.sum are tidy
• AGENTS.md: Update make check description to reflect actual behavior.

[Copilot]

Pull request overview

Refactors Go linting and dev-tooling to support PR-incremental linting and add module tidiness checks, aiming for more flexible local/CI workflows.

Changes:

• Moves “new issues only” golangci-lint behavior from .golangci.yaml into make/CI via --new-from-rev.
• Adds go mod tidy/go mod tidy -diff to fix/check to keep go.mod/go.sum tidy.
• Updates devcontainer and CI to download module deps earlier.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Makefile	Adds NEW_FROM_REV and applies --new-from-rev to lint/fix; adds go mod tidy and a tidy diff check.
AGENTS.md	Updates make check description wording (but command examples now need alignment with Makefile changes).
.golangci.yaml	Removes config-based incremental lint settings so CLI can control it.
.github/workflows/pr-check.yml	Passes NEW_FROM_REV=origin/${{ github.base_ref }} to limit lint to PR-introduced issues.
.devcontainer/post_create.sh	Adds go mod download before installing Go dev tools.

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