Add comprehensive GitHub Copilot instructions for the g CLI tool

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in the g repository.

What's Included

The instructions cover all essential aspects of working with the codebase:

Build & Development Workflow:

• Validated build commands with exact timing measurements (12s initial build, 1.5s subsequent)
• Build variants (lite: 7.4MB, full: 8.1MB) with Go build tags for optional features
• Required tooling setup (gofumpt, golangci-lint) with installation commands
• Complete development workflow with timeout specifications

Testing & Validation:

• Test suite execution (11 seconds, all validated)
• Manual validation scenarios with real file operations
• End-to-end user scenarios for tree view, table format, JSON output
• Git integration testing in different environments

CI/CD Requirements:

• Code formatting requirements (gofumpt mandatory for CI)
• Linting validation (golangci-lint with 3-minute timeout)
• Multi-platform build verification matching GitHub workflows

Critical Timing Information:

• NEVER CANCEL warnings with specific timeout recommendations
• Complete workflow timing: ~90 seconds total
• Performance expectations for different operations

Manual Testing Performed

All commands and scenarios in the instructions have been validated:

# Build variants tested
✅ Standard build: go build .
✅ Lite build: CGO_ENABLED=0 go build -ldflags="-s -w" -o g-lite .
✅ Full build: CGO_ENABLED=0 go build -ldflags="-s -w" -tags="fuzzy mounts" -o g-full .

# Functionality validation
✅ Tree view with icons and sizes
✅ Table format with timestamps  
✅ JSON output validation with jq
✅ Git integration behavior
✅ Shell integration (bash/zsh/fish aliases)

# Development tools
✅ Code formatting with gofumpt
✅ Linting with golangci-lint
✅ Complete test suite execution

The instructions follow the imperative tone requirement and include exhaustive validation steps to ensure any GitHub Copilot agent can successfully work in this codebase from a fresh clone.

Fixes #261.

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[Copilot]

Pull Request Overview

This PR adds comprehensive GitHub Copilot coding agent instructions to enhance developer experience and productivity when working with the g CLI tool repository. The instructions provide detailed guidance on build processes, testing workflows, and development best practices.

• Adds comprehensive .github/copilot-instructions.md with detailed development workflows
• Updates multiple Go files to use explicit return statements for better code clarity
• Includes validated build commands, timing expectations, and CI/CD requirements

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
.github/copilot-instructions.md	Comprehensive Copilot instructions covering build, test, and development workflows
internal/theme/color.go	Makes return statements explicit in three functions
internal/git/git_status.go	Makes return statement explicit in ParseShort function
internal/filter/itemfliter.go	Makes return statements explicit in two functions
internal/display/printer.go	Makes return statement explicit in prefixAndName function

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