feat(core): #137 add pre/post-turn snapshot hooks

Wire `pre_turn_snapshot` and `post_turn_snapshot` helpers into
`core::turn`, then call them from `Engine::handle_send_message` —
pre-turn fires right after `turn_counter` is incremented, post-turn
fires right after `Event::TurnComplete` is emitted.

Both hooks are dispatched via `tokio::task::spawn_blocking` so the
agent loop never waits on the side-git commit, and helper failures are
swallowed at WARN log level so a busted disk or missing `git` binary
can never derail a turn (per the snapshot module's documented
non-fatal contract).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/tui/src/core/engine.rs

@@ -61,7 +61,7 @@ use super::events::{Event, TurnOutcomeStatus};
 use super::ops::Op;
 use super::session::Session;
 use super::tool_parser;
-use super::turn::{TurnContext, TurnToolCall};
+use super::turn::{TurnContext, TurnToolCall, post_turn_snapshot, pre_turn_snapshot};
 
 // === Types ===
 
@@ -1436,6 +1436,15 @@ impl Engine {
         self.turn_counter = self.turn_counter.saturating_add(1);
         self.capacity_controller.mark_turn_start(self.turn_counter);
 
+        // Snapshot the workspace BEFORE we touch a single tool. Spawn on
+        // the blocking pool so the agent loop never waits on the side-git
+        // commit; failure is non-fatal (the helper logs at WARN).
+        let pre_workspace = self.session.workspace.clone();
+        let pre_seq = self.turn_counter;
+        tokio::task::spawn_blocking(move || {
+            let _ = pre_turn_snapshot(&pre_workspace, pre_seq);
+        });
+
         // Emit turn started event
         let _ = self
             .tx_event
@@ -1656,6 +1665,14 @@ impl Engine {
             })
             .await;
 
+        // Post-turn snapshot. Same non-blocking, non-fatal contract as
+        // the pre-turn hook above.
+        let post_workspace = self.session.workspace.clone();
+        let post_seq = self.turn_counter;
+        tokio::task::spawn_blocking(move || {
+            let _ = post_turn_snapshot(&post_workspace, post_seq);
+        });
+
         // Checkpoint-restart cycle boundary (issue #124). The turn just
         // settled cleanly — no in-flight tools, no streaming, no pending
         // approval — so this is the safe phase to swap the context if we've

crates/tui/src/core/turn.rs

@@ -2,8 +2,20 @@
 //!
 //! A "turn" is one user message and the resulting AI response,
 //! including any tool calls that occur.
+//!
+//! ## Snapshot lifecycle hooks
+//!
+//! [`pre_turn_snapshot`] and [`post_turn_snapshot`] book-end a turn by
+//! taking a workspace-level snapshot into a side git repo (see
+//! `crate::snapshot`). They are intentionally non-blocking and
+//! non-fatal: any IO error is logged at WARN and swallowed so a busted
+//! filesystem or missing `git` binary never derails the agent loop.
+//! `/restore N` and the `revert_turn` tool both consume these
+//! snapshots.
 
 use crate::models::Usage;
+use crate::snapshot::SnapshotRepo;
+use std::path::Path;
 use std::time::{Duration, Instant};
 
 /// Context for a single turn (user message + AI response).
@@ -116,6 +128,36 @@ fn add_optional_usage(total: Option<u32>, delta: Option<u32>) -> Option<u32> {
     }
 }
 
+/// Take a `pre-turn:<seq>` workspace snapshot.
+///
+/// Returns the snapshot SHA on success, `None` on any error. Errors are
+/// logged at WARN; the turn loop must not block on this.
+pub fn pre_turn_snapshot(workspace: &Path, turn_seq: u64) -> Option<String> {
+    snapshot_with_label(workspace, &format!("pre-turn:{turn_seq}"))
+}
+
+/// Take a `post-turn:<seq>` workspace snapshot. Same failure model as
+/// [`pre_turn_snapshot`].
+pub fn post_turn_snapshot(workspace: &Path, turn_seq: u64) -> Option<String> {
+    snapshot_with_label(workspace, &format!("post-turn:{turn_seq}"))
+}
+
+fn snapshot_with_label(workspace: &Path, label: &str) -> Option<String> {
+    match SnapshotRepo::open_or_init(workspace) {
+        Ok(repo) => match repo.snapshot(label) {
+            Ok(id) => Some(id.0),
+            Err(e) => {
+                tracing::warn!(target: "snapshot", "snapshot '{label}' failed: {e}");
+                None
+            }
+        },
+        Err(e) => {
+            tracing::warn!(target: "snapshot", "snapshot repo init failed: {e}");
+            None
+        }
+    }
+}
+
 impl TurnToolCall {
     /// Create a new tool call record
     pub fn new(id: String, name: String, input: serde_json::Value) -> Self {