feat: phase 2 — sessions export + rétention, init --template (closes #52 #53)

Axe 3 (#52) : templates embarqués dans le binaire (web, python, rust, cli)
- am init --template <stack> génère une config pré-remplie : groupe dev,
  profil par défaut, hooks projet (npm install / uv sync / cargo fetch)
- substitution {project} {dir} au rendu ; --template list pour lister
- fallback détection automatique de stack quand aucun template
- config générée validée (parse YAML + validate) par les tests

Axe 4 (#53) : am sessions export + rétention configurable
- am sessions --export <id> [--output <fichier>] : JSON complet et
  reproductible (métadonnées + commandes history/<id>.jsonl + extrait de
  log 4 Ko + rendu lisible 'readable'), compatible archivage manuel
- settings.sessions_retention_days (défaut 90) : purge au démarrage
  (silencieuse) et am sessions --retention <jours>
- purge limitée à l'index dérivé sessions.json — le journal d'événements
  primaire n'est jamais touché ; EventKind::Prune journalisé (visibilité
  am audit)
- sessions en cours (running) jamais purgées ; erreur claire si id inconnu

REPL : sessions --export/--retention/--output + init --template ; help
specs, tip cheat sheet, config.yaml, README, man pages régénérées.
Version 0.4.7, ROADMAP cochée, 276 tests verts.

src/templates.rs

@@ -0,0 +1,138 @@
+//! Embedded project templates for 'am init --template' (issue #52).
+//! Each template renders an agent-manager.yaml pre-filled for a stack:
+//! a dev group, a default profile and project hooks. Variables {project}
+//! and {dir} are substituted at render time.
+
+use anyhow::{bail, Result};
+
+/// (name, description) of every available template.
+pub const TEMPLATES: &[(&str, &str)] = &[
+    ("web", "frontend/web full-stack : node, npm, tests, serveur local"),
+    ("python", "python : uv/pip, pytest, ruff, mypy"),
+    ("rust", "rust : cargo, clippy, tests, rustfmt"),
+    ("cli", "générique CLI : commandes shell, scripts, git"),
+];
+
+/// The four embedded templates.
+const WEB: &str = r#"# Généré par 'am init --template web' — projet {project}
+# Répertoire : {dir}
+version: "1.0"
+groups:
+  dev: [claude-code, opencode, gemini-cli]
+profiles:
+  dev:
+    agent: claude-code
+    env:
+      NODE_ENV: development
+      CI: "1"
+    args: []
+settings:
+  hooks:
+    on_install: ["npm install"]
+    on_start: ["npm run dev"]
+agents: []
+"#;
+
+const PYTHON: &str = r#"# Généré par 'am init --template python' — projet {project}
+# Répertoire : {dir}
+version: "1.0"
+groups:
+  dev: [claude-code, codex, aider]
+profiles:
+  dev:
+    agent: claude-code
+    env:
+      PYTHONUNBUFFERED: "1"
+      PYTHONDONTWRITEBYTECODE: "1"
+    args: []
+settings:
+  hooks:
+    on_install: ["uv sync --all-extras"]
+    on_start: ["uv run pytest -q"]
+agents: []
+"#;
+
+const RUST: &str = r#"# Généré par 'am init --template rust' — projet {project}
+# Répertoire : {dir}
+version: "1.0"
+groups:
+  dev: [claude-code, codex, jcode]
+profiles:
+  dev:
+    agent: claude-code
+    env:
+      CARGO_TERM_COLOR: always
+      RUST_BACKTRACE: "1"
+    args: []
+settings:
+  hooks:
+    on_install: ["cargo fetch"]
+    on_start: ["cargo test"]
+agents: []
+"#;
+
+const CLI: &str = r#"# Généré par 'am init --template cli' — projet {project}
+# Répertoire : {dir}
+version: "1.0"
+groups:
+  dev: [claude-code, opencode]
+profiles:
+  dev:
+    agent: claude-code
+    env: {}
+    args: []
+agents: []
+"#;
+
+fn template_of(name: &str) -> Option<&'static str> {
+    match name {
+        "web" => Some(WEB),
+        "python" => Some(PYTHON),
+        "rust" => Some(RUST),
+        "cli" => Some(CLI),
+        _ => None,
+    }
+}
+
+/// Render a template for a project. `project` is the project name (used in
+/// the header comment), `dir` the absolute directory path. Unknown template
+/// names fail with the available list.
+pub fn render(name: &str, project: &str, dir: &str) -> Result<String> {
+    let Some(tpl) = template_of(name) else {
+        let list = TEMPLATES
+            .iter()
+            .map(|(n, _)| *n)
+            .collect::<Vec<_>>()
+            .join(", ");
+        bail!("unknown template '{name}' — available: {list}");
+    };
+    Ok(tpl.replace("{project}", project).replace("{dir}", dir))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn renders_all_templates_with_substitution() {
+        for (name, _) in TEMPLATES {
+            let out = render(name, "mon-app", "/work/mon-app").unwrap();
+            assert!(out.contains("mon-app"), "{name}: project substituted");
+            assert!(out.contains("/work/mon-app"), "{name}: dir substituted");
+            assert!(!out.contains("{project}"), "{name}: no leftover var");
+            assert!(!out.contains("{dir}"), "{name}: no leftover var");
+            // The rendered config must parse and validate.
+            let cfg: crate::config::Config = serde_yaml::from_str(&out).unwrap_or_else(|e| {
+                panic!("{name}: invalid YAML: {e}\n{out}")
+            });
+            assert!(crate::config::validate(&cfg).is_empty(), "{name}: invalid config");
+            assert!(!cfg.groups.is_empty(), "{name}: a dev group is expected");
+        }
+    }
+
+    #[test]
+    fn unknown_template_fails_with_list() {
+        let err = render("cobol", "x", "/tmp").unwrap_err();
+        assert!(err.to_string().contains("web"), "{err}");
+    }
+}