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

ai/prompts.go

@@ -0,0 +1,10 @@
+package ai
+
+import "fmt"
+
+const SystemPrompt = "You are a concise shell command completion assistant. Provide ONLY the completed shell command line. Do not explain, do not use markdown formatting, and do not wrap the command in code blocks or backticks. Always ensure valid shell syntax: if an argument contains spaces or parentheses (such as git commit messages), you MUST wrap that argument in double quotes \"...\"."
+
+func BuildCompletionPrompt(buf string, env EnvSnapshot, dynamicCtx string) string {
+	return fmt.Sprintf("Complete this shell command line: %s\nContext:\nCwd: %s\nLastCmd: %s\nLastExitCode: %d\nGitStatus: %s\nRecentCmds: %v\nDynamicContext: %s",
+		buf, env.Cwd, env.LastCmd, env.LastExitCode, env.GitStatus, env.RecentCmds, dynamicCtx)
+}