chore: improve docs, tooling, tests, and dev environment

[min0625]

Summary

This PR improves documentation, developer tooling, tests, and dev environment setup across the repository.

Changes

Documentation

• Add GoDoc comments to all exported methods (Go, TryGo, Wait, SetLimit) explaining panic recovery behaviour
• Add inline comments in Wait() explaining the panicValue channel logic
• Update README.md with usage examples
• Add x/errgroup/README.md
• Add AGENTS.md with development conventions for AI coding agents

Tooling

• Add mise.toml for pinned tool versions (Go 1.24.x, golangci-lint 2.x)
• Simplify Makefile by removing asdf/install dependencies
• Remove deprecated .tool-versions

Dev Environment

• Add .devcontainer/ for reproducible dev container setup
• Add .vscode/ editor settings

CI

• Add .github/workflows/pr-check.yml CI workflow
• Remove deprecated .github/workflows/go.yml

Tests

• Expand errgroup_test.go with additional test cases (Test_Group_TryGo, Test_Group_TryGo_Panic)
• Expand x/errgroup/errgroup_test.go with more test cases (Test_WithContext_ZeroValue, Test_Group_TryGo_ContextPropagation, Test_Group_TryGo_Panic)

Verification

All tests pass with the race detector enabled and no lint issues:

make check  # lint + test -race -failfast ./...

[Copilot]

Pull request overview

This PR updates the repository’s developer experience and documentation around the panic-recovering errgroup implementations (root and x/errgroup), while also modernizing tooling (mise), CI, and expanding test coverage for TryGo and panic behavior.

Changes:

• Document panic recovery and zero-value behavior across packages (GoDoc + READMEs + AGENTS.md).
• Introduce mise-pinned tooling/devcontainer/VSCode setup and update CI workflow to run make check.
• Expand tests for TryGo (limit behavior, context propagation in x/errgroup, panic paths).

Reviewed changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
errgroup.go	Adds/clarifies exported doc comments for panic propagation and Wait implementation notes.
panic.go	Documents Recovered and Stack fields for panic wrapper types.
errgroup_test.go	Fixes an assertion signature and adds TryGo + panic tests.
README.md	Adds package table + panic behavior docs + improves example error handling.
go.mod	Bumps Go directive and updates golang.org/x/sync.
go.sum	Updates checksums for the x/sync bump.
mise.toml	Pins tool versions (Go, golangci-lint).
Makefile	Simplifies targets and removes asdf/install dependencies from common targets.
AGENTS.md	Adds repo conventions for AI agents (tooling/testing/panic behavior).
x/errgroup/errgroup.go	Enhances package/type/method docs to clarify context + panic behavior.
x/errgroup/errgroup_test.go	Fixes assertion signatures and adds TryGo tests (including context propagation + panic).
x/errgroup/README.md	Adds README for the context-aware variant with examples and behavioral notes.
.github/workflows/pr-check.yml	Adds PR workflow to run make check.
.github/workflows/go.yml	Removes deprecated workflow.
.tool-versions	Removes deprecated asdf tool version pinning.
.devcontainer/devcontainer.json	Adds devcontainer configuration using mise feature.
.devcontainer/post_create.sh	Installs tools and configures shell initialization for mise.
.vscode/settings.json	Adds Go + golangci-lint formatting/lint settings.
.vscode/launch.json	Adds debug configurations for tests and workspace debugging.

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