feat: Allow resuming previous AI sessions (#3407)

This PR introduces session continuation to Atuin AI.

* Conversations with Atuin AI are stored in a local SQLite database
* Upon startup, Atuin AI tries to find a session to resume based on its
directory/workspace and the time since the last event
* If found, Atuin AI will show a note that the session has been resumed,
and an event is added to help the LLM know where the invocation
boundaries are
* If not, Atuin AI will create a new conversation
* The user can create a new conversation with `/new`
* The new setting `ai.session_continue_minutes`, which defaults to `60`,
controls how old the last event in a session can be before it's no
longer considered for automatic resuming.

<img width="1055" height="593" alt="image"
src="https://github.com/user-attachments/assets/3f9ff01a-ef64-44a9-b0e2-3a4252c5746f"
/>

## Architecture

A new `SessionService` trait defines an API contract for a service that
can manage session data. `LocalSessionService` implements this, with
`DaemonSessionService` a possible future extension point.

`SessionManager` owns a `dyn SessionService` and delegates as
appropriate.

3 files changed, 65 insertions, 1 deletions

diff --git a/crates/atuin-ai/src/tui/components/input_box.rs b/crates/atuin-ai/src/tui/components/input_box.rs
index f5e0fe2b..6e041418 100644
--- a/crates/atuin-ai/src/tui/components/input_box.rs
+++ b/crates/atuin-ai/src/tui/components/input_box.rs
@@ -19,7 +19,7 @@ use ratatui_core::{
 };
 use tui_textarea::TextArea;
 
-use crate::tui::events::AiTuiEvent;
+use crate::tui::{events::AiTuiEvent, slash::SlashCommandSearchResult};
 
 /// A bordered text input box backed by tui-textarea.
 ///
@@ -35,6 +35,8 @@ pub(crate) struct InputBox {
     pub footer: String,
     /// Whether the input is currently active (shows cursor, accepts input)
     pub active: bool,
+    /// If the user has typed a slash command, this holds the best match for it.
+    pub slash_suggestion: Option<SlashCommandSearchResult>,
 }
 
 pub(crate) struct InputBoxState {
@@ -129,6 +131,18 @@ fn input_box(
                     textarea.insert_newline();
                     return EventResult::Consumed;
                 }
+                crossterm::event::KeyCode::Tab if props.slash_suggestion.is_some() => {
+                    // If there's a slash command suggestion, Tab accepts it.
+                    if let Some(suggestion) = &props.slash_suggestion {
+                        textarea.clear();
+                        textarea.insert_str(format!("/{}", suggestion.command.name));
+                        // Manually trigger an input update event so the slash suggestion box can update immediately
+                        if let Some(ref tx) = state.tx {
+                            let _ = tx.send(AiTuiEvent::InputUpdated(textarea.lines().join("\n")));
+                        }
+                        return EventResult::Consumed;
+                    }
+                }
                 crossterm::event::KeyCode::Enter => {
                     if key.modifiers.contains(KeyModifiers::SHIFT) {
                         textarea.insert_newline();
diff --git a/crates/atuin-ai/src/tui/components/mod.rs b/crates/atuin-ai/src/tui/components/mod.rs
index 3458327d..9959dbad 100644
--- a/crates/atuin-ai/src/tui/components/mod.rs
+++ b/crates/atuin-ai/src/tui/components/mod.rs
@@ -2,3 +2,4 @@ pub(crate) mod atuin_ai;
 pub(crate) mod input_box;
 pub(crate) mod markdown;
 pub(crate) mod select;
+pub(crate) mod session_continue;
diff --git a/crates/atuin-ai/src/tui/components/session_continue.rs b/crates/atuin-ai/src/tui/components/session_continue.rs
new file mode 100644
index 00000000..bfbfb191
--- /dev/null
+++ b/crates/atuin-ai/src/tui/components/session_continue.rs
@@ -0,0 +1,49 @@
+use chrono_humanize::HumanTime;
+use eye_declare::{Elements, Hooks, Span, Text, component, element, props};
+use ratatui::style::{Color, Modifier, Style};
+
+#[props]
+pub(crate) struct SessionContinue {
+    pub continued_at: Option<chrono::DateTime<chrono::Utc>>,
+}
+
+#[derive(Default)]
+pub(crate) struct SessionContinueState {
+    /// Frozen on mount so the label doesn't change on every render.
+    label: Option<String>,
+}
+
+#[component(props = SessionContinue, state = SessionContinueState)]
+fn session_continue(
+    _props: &SessionContinue,
+    state: &SessionContinueState,
+    hooks: &mut Hooks<SessionContinue, SessionContinueState>,
+) -> Elements {
+    hooks.use_mount(|props, state| {
+        state.label = Some(match props.continued_at {
+            Some(t) => {
+                let human = HumanTime::from(t - chrono::Utc::now());
+                format!(
+                    "  Continuing previous session (last active {human}) - type /new to start a new session"
+                )
+            }
+            None => {
+                "  Continuing previous session - type /new to start a new session".to_string()
+            }
+        });
+    });
+
+    let resume_label = state
+        .label
+        .as_deref()
+        .unwrap_or("  Continuing previous session - type /new to start a new session");
+
+    element! {
+        Text {
+            Span(
+                text: resume_label,
+                style: Style::default().fg(Color::DarkGray).add_modifier(Modifier::ITALIC),
+            )
+        }
+    }
+}