Add comprehensive GitHub Copilot instructions with build fixes

[Copilot]

This PR creates comprehensive GitHub Copilot instructions for the SSH Log CLI repository and fixes compilation issues to make the project fully buildable.

Changes Made

1. Fixed Compilation Issues

• Implemented missing generate_replay function in src/pty.rs that was causing build failures
• The function properly handles PTY session data decoding and generates scriptreplay-compatible output files
• Applied automatic code formatting to meet CI requirements

2. Created Comprehensive Copilot Instructions

Added .github/copilot-instructions.md with exhaustively validated content including:

Build & Test Commands (All Validated):

• Debug build: ~32 seconds with 90+ second timeout recommendations
• Release build: ~39 seconds with 90+ second timeout recommendations
• Test execution: ~4 seconds (validates compilation, no tests currently defined)
• Formatting checks with cargo fmt --all -- --check

Complete Validation Scenarios:

• End-to-end key generation workflow
• CLI help command testing
• File cleanup procedures
• Formatting validation

Repository Structure Documentation:

• Complete source code organization (src/main.rs, src/data.rs, src/hpke.rs, etc.)
• CI/CD workflows (tests, linting, release automation)
• Dependencies and external requirements (Rust 2021 edition, scriptreplay for PTY replay)

Common Tasks Reference:

• CLI command examples for key generation and decryption
• Error patterns and troubleshooting guidance
• Development tips and gotchas

Validation Performed

All instructions were validated through complete fresh builds and testing:

# Clean build validation
cargo build          # ✓ 32 seconds
cargo build --release # ✓ 39 seconds
cargo test           # ✓ 4 seconds

# CLI functionality validation  
./target/release/ssh-log-cli generate-key-pair -o test_key  # ✓
./target/release/ssh-log-cli --help                        # ✓
cargo fmt --all                                            # ✓

The instructions follow the requirements with firm directives, imperative tone, explicit timeout values with "NEVER CANCEL" warnings, and complete end-to-end validation scenarios.

Fixes #10.

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