feat: Implement SCP server protocol

[inureyes]

Summary

• Add SCP server handler with sink and source modes for file transfers
• Implement SCP command parsing with support for -t, -f, -r, -p, -d, -v flags
• Integrate SCP handling with exec_request for automatic command detection
• Add comprehensive path traversal prevention for security

Implementation Details

SCP Protocol Handler (src/server/scp.rs)

• ScpCommand: Parser for SCP command flags and arguments
• ScpHandler: Main handler implementing the SCP protocol
  • Sink mode (-t): Receives files from the client
  • Source mode (-f): Sends files to the client
  • Recursive transfer (-r): Support for directory transfers
  • Time preservation (-p): Preserves file modification times

Security Features

• Path traversal prevention (files cannot escape root directory)
• Filename validation (no /, .., or . as filename)
• File size limit (10GB max)
• Symlink handling in recursive transfers (skipped for security)

Configuration

• New scp_enabled setting in ServerConfig (default: true)
• Builder method scp_enabled() for programmatic configuration
• YAML configuration support via scp.enabled

Test Plan

• Unit tests for SCP command parsing
• Unit tests for path resolution and traversal prevention
• Unit tests for handler creation and configuration
• All existing tests pass
• Manual testing with scp client (requires running server)

Closes #133

[inureyes]

PR Finalization Report

Project Structure Discovered

• Project Type: Rust (Cargo.toml)
• Test Framework: cargo test (standard Rust testing)
• Documentation System: Plain markdown in docs/architecture/
• Multi-language Docs: No (English only)
• Lint Tools: cargo fmt, cargo clippy

Checklist

Tests

• Analyzed existing test structure (16 SCP-related tests)
• All existing tests passing (933 tests total)
• No additional tests needed (comprehensive coverage already exists)

Documentation

• ARCHITECTURE.md updated - Added SCP handler to SSH Server Module section and file structure
• docs/architecture/server-configuration.md updated - Added comprehensive SCP Protocol Handler section
• docs/architecture/README.md updated - Added SCP Handler reference in Server Components

Code Quality

• cargo fmt: All files formatted
• cargo clippy: Fixed while_let_loop warning in scp.rs
• All warnings resolved

Changes Made

1. src/server/scp.rs: Refactored loop { match ... } to while let pattern to satisfy clippy
2. src/server/handler.rs: Applied cargo fmt formatting
3. ARCHITECTURE.md: Added ScpHandler documentation with security features
4. docs/architecture/server-configuration.md: Added SCP Protocol Handler section with protocol overview, operation modes, flags, security features, configuration, and usage examples
5. docs/architecture/README.md: Added SCP Handler to Server Components list

Verification Results

cargo fmt --check: OK
cargo clippy -- -D warnings: OK  
cargo test --lib: 933 passed, 0 failed

All checks passing. Ready for merge.