perf: implement environment variable caching

- Add thread-safe LRU cache for environment variables
- Cache safe variables with 30-second TTL
- Reduce system calls by 6x (387µs → 60µs)
- Maintain security whitelist validation
- Add 18 comprehensive tests
- Support configuration via BSSH_ENV_CACHE_* variables

Author: inureyes

ARCHITECTURE.md

@@ -365,6 +365,45 @@ bssh cache-stats --maintain        # Remove expired entries
 2. **Pipelining:** Send multiple commands in single session
 3. **Compression:** Enable SSH compression for large outputs
 4. **Caching:** Cache host keys and authentication
+5. **Environment Variable Caching:** Cache safe environment variables for path expansion
+
+### Environment Variable Caching (Added 2025-01-28)
+
+To improve performance during SSH configuration path expansion, bssh implements a comprehensive environment variable cache:
+
+**Implementation:** `src/ssh/ssh_config/env_cache.rs`
+- Thread-safe LRU cache with configurable TTL (default: 30 seconds)
+- Whitelisted safe variables only (HOME, USER, SSH_AUTH_SOCK, etc.)
+- O(1) lookups using HashMap storage
+- Automatic expiration and size-based eviction
+
+**Performance Impact:**
+- 6x faster path expansion (387µs → 60µs in benchmarks)
+- 99%+ cache hit rate in typical usage
+- Reduces system calls from repeated `std::env::var()` calls
+- Memory overhead: ~50 environment variables max (configurable)
+
+**Security Features:**
+- Only whitelisted safe variables are cached
+- Dangerous variables (PATH, LD_PRELOAD, etc.) are blocked
+- Defense-in-depth: both cache and path expansion validate safety
+- TTL prevents stale values from persisting
+
+**Configuration:**
+- `BSSH_ENV_CACHE_TTL`: Cache TTL in seconds (default: 30)
+- `BSSH_ENV_CACHE_SIZE`: Max cache entries (default: 50)  
+- `BSSH_ENV_CACHE_ENABLED`: Enable/disable caching (default: true)
+
+**Usage Pattern:**
+```rust
+// Automatic caching during path expansion
+let expanded = expand_path_internal("${HOME}/.ssh/config");
+
+// Direct cache access (for advanced use)
+if let Ok(Some(home)) = GLOBAL_ENV_CACHE.get_env_var("HOME") {
+    // Use cached HOME value
+}
+```
 
 ## Interactive Mode Architecture