feat: Send user-defined context with `TERMINAL.md` (#3443)

This PR adds the ability to inject user-defined content into Atuin AI
requests, a la `AGENTS.md` or `CLAUDE.md`.

* `.atuin/TERMINAL.md` (or alternatively just `TERMINAL.md`) is checked in every directory from the cwd up to the root
* `~/.config/atuin/TERMINAL.md` (or equivalent config dir) is also
checked
* Supports Claude-style ``` !`` ``` and ```` ```!...``` ```` style shell
interpolation

1 files changed, 68 insertions, 0 deletions

diff --git a/crates/atuin-ai/src/user_context/mod.rs b/crates/atuin-ai/src/user_context/mod.rs
new file mode 100644
index 00000000..295efdec
--- /dev/null
+++ b/crates/atuin-ai/src/user_context/mod.rs
@@ -0,0 +1,68 @@
+//! User-authored context files (`TERMINAL.md`).
+//!
+//! Context files are markdown documents that can embed shell commands for
+//! dynamic content. Before each API request, context files are discovered
+//! by walking the filesystem, commands are executed, and the interpolated
+//! content is sent to the server as `config.user_contexts`.
+
+mod interpolate;
+mod walker;
+
+use std::path::Path;
+
+pub(crate) use walker::global_context_path;
+
+/// A fully resolved user context, ready to include in an API request.
+#[derive(Debug, Clone, serde::Serialize)]
+pub(crate) struct UserContext {
+    /// The path to the context file on disk.
+    pub path: String,
+    /// The interpolated content.
+    pub data: String,
+}
+
+/// Discover context files and interpolate embedded commands.
+///
+/// Walks from `start` up to the filesystem root looking for
+/// `.atuin/ai-context.md`, then checks `global_path`. Returns contexts
+/// ordered from most general (global/root) to most specific (deepest).
+pub(crate) async fn gather(
+    start: &Path,
+    global_path: Option<&Path>,
+    shell: &str,
+) -> Vec<UserContext> {
+    let raw_files = match walker::walk(start, global_path).await {
+        Ok(files) => files,
+        Err(e) => {
+            tracing::warn!("Failed to walk for context files: {e}");
+            return Vec::new();
+        }
+    };
+
+    if raw_files.is_empty() {
+        return Vec::new();
+    }
+
+    // Interpolate all files in parallel.
+    let mut handles = Vec::with_capacity(raw_files.len());
+    for file in raw_files {
+        let shell = shell.to_string();
+        handles.push(tokio::spawn(async move {
+            let data = interpolate::interpolate(&file.content, &shell).await;
+            UserContext {
+                path: file.path.to_string_lossy().to_string(),
+                data,
+            }
+        }));
+    }
+
+    let mut contexts = Vec::with_capacity(handles.len());
+    for handle in handles {
+        match handle.await {
+            Ok(ctx) => contexts.push(ctx),
+            Err(e) => tracing::warn!("Context interpolation task failed: {e}"),
+        }
+    }
+
+    contexts
+}