feat(app): App::fetch doc mode with budget (fb-35)

Walks CanonicalDocument blocks, serializes to markdown, applies
chars/4 budget when opts.max_tokens is set. doc_not_found
preserved through StructuredError.

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

crates/kebab-app/src/fetch.rs

@@ -104,9 +104,67 @@ fn fetch_chunk(app: &App, id: ChunkId, opts: FetchOpts) -> Result<FetchResult> {
     })
 }
 
-fn fetch_doc(_app: &App, _id: DocumentId, _opts: FetchOpts) -> Result<FetchResult> {
-    // Implemented in Task 4.
-    anyhow::bail!("fetch_doc not yet implemented")
+fn fetch_doc(app: &App, id: DocumentId, opts: FetchOpts) -> Result<FetchResult> {
+    let doc = <kebab_store_sqlite::SqliteStore as DocumentStore>::get_document(&app.sqlite, &id)?
+        .ok_or_else(|| {
+            anyhow::Error::new(StructuredError(ErrorV1 {
+                schema_version: ERROR_V1_ID.to_string(),
+                code: "doc_not_found".to_string(),
+                message: format!("doc_id '{}' not found", id.0),
+                details: serde_json::Value::Null,
+                hint: None,
+            }))
+        })?;
+
+    let mut text = fmt_canonical_to_markdown(&doc);
+    let mut truncated = false;
+    if let Some(max_tokens) = opts.max_tokens {
+        let max_chars = max_tokens.saturating_mul(4);
+        if text.chars().count() > max_chars {
+            text = trim_to_chars(&text, max_chars);
+            truncated = true;
+        }
+    }
+
+    let now = OffsetDateTime::now_utc();
+    let stale = compute_stale(
+        doc_metadata_updated_at(&doc),
+        now,
+        app.config.search.stale_threshold_days,
+    );
+
+    Ok(FetchResult {
+        kind: FetchKind::Doc,
+        doc_id: doc.doc_id.clone(),
+        doc_path: doc.workspace_path.clone(),
+        indexed_at: doc_metadata_updated_at(&doc),
+        stale,
+        chunk: None,
+        context_before: Vec::new(),
+        context_after: Vec::new(),
+        text: Some(text),
+        line_start: None,
+        line_end: None,
+        effective_end: None,
+        truncated,
+    })
+}
+
+/// p9-fb-35: trim string to N chars (Unicode-safe). Mirrors fb-34's
+/// helper at `crates/kebab-app/src/app.rs` — kept local to avoid
+/// re-exporting an internal helper.
+fn trim_to_chars(s: &str, n: usize) -> String {
+    if s.chars().count() <= n {
+        return s.to_string();
+    }
+    let mut out = String::with_capacity(n * 4);
+    for (i, c) in s.chars().enumerate() {
+        if i >= n {
+            break;
+        }
+        out.push(c);
+    }
+    out
 }
 
 fn fetch_span(
@@ -170,11 +228,6 @@ fn doc_metadata_updated_at(doc: &CanonicalDocument) -> OffsetDateTime {
 /// flatten to plain text via the already-flattened `TextBlock.text`
 /// field. Good enough for an agent reading verbatim context. Used by
 /// Task 4 (doc mode) and Task 5 (span mode).
-//
-// The first caller lands in Task 4 (`fetch_doc`); silence the
-// stop-gap dead-code warning until then so this Task 3 commit lands
-// with a clean clippy run.
-#[allow(dead_code)]
 pub(crate) fn fmt_canonical_to_markdown(doc: &CanonicalDocument) -> String {
     let mut out = String::with_capacity(1024);
     for (i, block) in doc.blocks.iter().enumerate() {

crates/kebab-app/tests/fetch_integration.rs

@@ -83,3 +83,73 @@ fn fetch_chunk_unknown_id_returns_chunk_not_found() {
         "expected chunk_not_found error, got: {msg}"
     );
 }
+
+#[test]
+fn fetch_doc_returns_serialized_markdown() {
+    let env = common::TestEnv::new();
+    let body = "# Heading One\n\nFirst paragraph.\n\n## Sub\n\nSecond.\n";
+    common::ingest_md(&env, "doc.md", body);
+    let app = env.app();
+
+    // Discover doc_id via search hit (avoids depending on list_docs API shape).
+    let q = kebab_core::SearchQuery {
+        text: "First".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let doc_id = hits[0].doc_id.clone();
+
+    let result = app
+        .fetch(FetchQuery::Doc(doc_id), FetchOpts::default())
+        .unwrap();
+    assert_eq!(result.kind, FetchKind::Doc);
+    let text = result.text.expect("doc text");
+    assert!(text.contains("Heading One"), "doc text contains heading: {text:?}");
+    assert!(text.contains("First paragraph"), "doc text contains body");
+    assert!(!result.truncated);
+}
+
+#[test]
+fn fetch_doc_unknown_id_returns_doc_not_found() {
+    let env = common::TestEnv::new();
+    let app = env.app();
+    let err = app
+        .fetch(
+            FetchQuery::Doc(kebab_core::DocumentId("nonexistent-doc".to_string())),
+            FetchOpts::default(),
+        )
+        .unwrap_err();
+    assert!(err.to_string().contains("doc_not_found"), "got: {err}");
+}
+
+#[test]
+fn fetch_doc_with_max_tokens_truncates() {
+    let env = common::TestEnv::new();
+    let p = "Lorem ipsum dolor sit amet consectetur adipiscing elit. ".repeat(20);
+    let body = format!("# Big\n\n{p}\n");
+    common::ingest_md(&env, "big.md", &body);
+    let app = env.app();
+    let q = kebab_core::SearchQuery {
+        text: "Lorem".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let doc_id = hits[0].doc_id.clone();
+
+    let result = app
+        .fetch(
+            FetchQuery::Doc(doc_id),
+            FetchOpts {
+                context: None,
+                max_tokens: Some(20), // ~80 chars
+            },
+        )
+        .unwrap();
+    assert!(result.truncated);
+    let text = result.text.expect("doc text");
+    assert!(text.chars().count() <= 100, "trimmed text len {}", text.chars().count());
+}