feat: am ai — langage naturel vers action shell securisee via AIChat (issues #96 #97)

- Commande am ai (alias CLI: am shell) : conversationnel par defaut
  (aichat -f <ctx> "<prompt>"), mode --exec avec generation aichat --dry-run
- Securite (issue #97) : classification safe/risky + certainite affichee,
  politique dry-run par defaut (settings.shell_ai.default_safety:
  dry-run | confirm | auto), confirmation y/N pour les commandes risky,
  --yes pour executer, --dry-run force la simulation
- Flags : --exec/-e, --files/-f (defaut: dossier courant), --provider,
  --model (registre providers -> env aichat + modele, token via keyring)
- Execution via le shell utilisateur (default_shell > SHELL > COMSPEC)
- Journalisation : EventKind::ShellAi (mode, risk, certainty, executed)
- Module src/shell_ai.rs (classification, politique, generation, execution)
- Câblage complet : cli, dispatch, help, REPL (parseur+completer+banner),
  tip, man pages (am-ai.1), i18n EN, config.yaml (bloc shell_ai)
- Tests : 7 unitaires (classification, certainty, extraction, decision)
  + 4 integration (shim aichat.cmd : dry-run par defaut, --yes, mode
  conversationnel, mode confirm) — 339 tests verts
- v1.0.5 ; ROADMAP axe 13.3 coche (6 lignes), epic #93 clôturee

closes #93
closes #96
closes #97

src/config.rs

@@ -162,6 +162,10 @@ pub struct Settings {
     /// (issue #89).
     #[serde(default)]
     pub providers: Option<BTreeMap<String, Option<ProviderDef>>>,
+    /// am ai security settings (issue #97): default safety policy and
+    /// extra risky command patterns for the Shell AI command.
+    #[serde(default)]
+    pub shell_ai: Option<ShellAiSettings>,
 }
 
 /// Anonymous opt-in telemetry (issue #76): aggregated counters only — never
@@ -200,6 +204,34 @@ impl Default for AskSettings {
     }
 }
 
+/// am ai security settings (issue #97): safety policy and risky patterns
+/// for the Shell AI command. The default policy is `dry-run` — nothing is
+/// executed without an explicit confirmation.
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+#[serde(default)]
+pub struct ShellAiSettings {
+    /// Default safety policy of `am ai --exec`:
+    /// `dry-run` (show only), `confirm` (ask for risky commands) or
+    /// `auto` (execute everything). Default: dry-run.
+    pub default_safety: Option<String>,
+    /// Extra command substrings classified as risky (in addition to the
+    /// built-in patterns: rm -rf, dd if, mkfs, chmod -R 777, ...).
+    #[serde(default)]
+    pub risky_patterns: Vec<String>,
+}
+
+impl ShellAiSettings {
+    /// The effective default safety mode (unknown values fall back to
+    /// dry-run — the safe choice).
+    pub fn safety_mode(&self) -> crate::shell_ai::SafetyMode {
+        match self.default_safety.as_deref() {
+            Some("auto") => crate::shell_ai::SafetyMode::Auto,
+            Some("confirm") => crate::shell_ai::SafetyMode::Confirm,
+            _ => crate::shell_ai::SafetyMode::DryRun,
+        }
+    }
+}
+
 /// Community registry settings (issue #77).
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
 #[serde(default)]
@@ -969,6 +1001,9 @@ pub fn merge(base: &mut Config, overlay: Config) {
     if o.default_provider.is_some() {
         s.default_provider = o.default_provider;
     }
+    if o.shell_ai.is_some() {
+        s.shell_ai = o.shell_ai;
+    }
     // Providers merge key by key (issue #88): an overlay adds or replaces
     // one provider without wiping the others. An explicit `null` in the
     // overlay DELETES the provider (the standard YAML overlay pattern) so a