feat: AI suggestion (#34)

## Feat
* feat(ai): add AI suggestion engine, context gathering, and ghost text
overlay (`v0.3.0`)

## Security & Perf
* fix(ai): restrict background `--help` execution to a hardcoded command
allowlist to prevent RCE
* fix(root): defer context cancellation in goroutine to prevent resource
leaks
* perf(ai): implement LRU eviction and maximum size limit for
`ProviderCache`
* perf(ai): truncate command context output
(`CommandContextProvider.Gather`) to 1000 characters to save tokens

## Bug Fixes
* fix(ai): add mutex synchronization and thread-safe snapshotting to
`AIEngine.RegisterProvider` and `GatherDynamicContext`
* fix(ai): return a copy of fresh `Suggestion` in `AIEngine.Suggest` to
prevent cache mutation
* fix(ai): use length-prefixed encoding in `EnvSnapshot.Hash` to prevent
delimiter collisions
* fix(ai): skip variable assignment lines containing `=` when extracting
Makefile targets
* fix(runner): check `scanner.Err()` and return `nil` on scan errors in
justfile generator
* test(ci): rename `git commit` to `git checkout` in overlay test to
resolve CI `typos` false positive

config/defaults.go

@@ -26,6 +26,18 @@ func DefaultConfig() *Config {
 			Channel:        "stable",
 			CheckInterval:  Duration(24 * time.Hour),
 		},
+		AI: AIConfig{
+			Enabled:       false,
+			Provider:      "",
+			DebounceMS:    500,
+			MinIntervalMS: 1000,
+			Providers:     nil,
+			SuggestOnEmpty: SuggestOnEmptyConfig{
+				Enabled:       false,
+				DebounceMS:    800,
+				MinIntervalMS: 5000,
+			},
+		},
 	}
 }