fix(dogfood): document-centric try_skip_unchanged for twin-file idempotency

Identical-content files at different workspace paths share one assets row
(assets.asset_id = blake3 content hash, PRIMARY KEY). The UPSERT
`ON CONFLICT(asset_id) DO UPDATE SET workspace_path = excluded` made
twin files overwrite each other's workspace_path on every ingest, so
`get_asset_by_workspace_path(path1)` returned the OTHER twin's row (or
None) — break idempotent unchanged-detection for both files.

Fix: switch try_skip_unchanged to document-centric lookup. `documents.
workspace_path` is already UNIQUE (V001) and `id_for_doc(path, ...)`
includes path, so each twin has its own stable document row. Compare
`doc.source_asset_id` with the new asset's checksum instead of going
through the assets table.

Dogfood (multi-root: kebab-docs + httpx + zod + lodash) showed 27 of
726 docs marked Updated on every idempotent re-ingest — all 27 are
twin-file victims (empty `__init__.py` ×3, AGENTS.md ↔ CLAUDE.md
same content, duplicate logo PDFs/JPGs).

After: re-ingest reports 0 new / 0 updated / 726 unchanged.

No schema migration needed.

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

crates/kebab-app/tests/twin_files_idempotent.rs

@@ -0,0 +1,90 @@
+//! Regression test for the twin-file idempotency bug.
+//!
+//! Identical-content files at different workspace paths share one
+//! `assets` row (`asset_id` = blake3 content hash, PRIMARY KEY). The
+//! old UPSERT `ON CONFLICT(asset_id) DO UPDATE SET workspace_path =
+//! excluded.workspace_path` made each twin overwrite the other's path
+//! on every ingest, so `get_asset_by_workspace_path(path1)` returned
+//! None (or the wrong twin) → re-process every time.
+//!
+//! Fix: `try_skip_unchanged` now uses `get_document_by_workspace_path`
+//! instead.  `documents.workspace_path` is UNIQUE (V001) so each twin
+//! has its own stable document row.
+//!
+//! Assertion contract:
+//!   1st ingest → 2 New (one per twin)
+//!   2nd ingest → 0 New, 0 Updated, 2 Unchanged
+
+mod common;
+
+use common::TestEnv;
+use kebab_app::ingest_with_config;
+use kebab_core::IngestItemKind;
+
+#[test]
+fn twin_files_second_ingest_is_unchanged() {
+    let env = TestEnv::lexical_only();
+
+    // Write two files with identical content at different paths.
+    let pkg_a = env.workspace_root.join("pkg_a");
+    let pkg_b = env.workspace_root.join("pkg_b");
+    std::fs::create_dir_all(&pkg_a).unwrap();
+    std::fs::create_dir_all(&pkg_b).unwrap();
+
+    let content = b"# shared\nThis content is identical in both files.\n";
+    std::fs::write(pkg_a.join("__init__.py"), content).unwrap();
+    std::fs::write(pkg_b.join("__init__.py"), content).unwrap();
+
+    // First ingest — both files must be New.
+    let first = ingest_with_config(env.config.clone(), env.scope(), false)
+        .expect("first ingest must succeed");
+    assert_eq!(first.errors, 0, "first ingest: no errors; report={first:?}");
+
+    let items = first.items.as_ref().expect("items must be present");
+    let twin_items: Vec<_> = items
+        .iter()
+        .filter(|i| {
+            i.doc_path.0.ends_with("__init__.py")
+        })
+        .collect();
+    assert_eq!(
+        twin_items.len(),
+        2,
+        "first ingest: expected exactly 2 __init__.py items; items={items:?}"
+    );
+    for item in &twin_items {
+        assert_eq!(
+            item.kind,
+            IngestItemKind::New,
+            "first ingest: each twin must be New; item={item:?}"
+        );
+    }
+
+    // Second ingest — same files, same content → both must be Unchanged.
+    let second = ingest_with_config(env.config.clone(), env.scope(), false)
+        .expect("second ingest must succeed");
+    assert_eq!(second.errors, 0, "second ingest: no errors; report={second:?}");
+    assert_eq!(second.new, 0, "second ingest: no new docs; report={second:?}");
+    assert_eq!(
+        second.updated, 0,
+        "second ingest: no updated docs (twin-file bug would set this to 2); report={second:?}"
+    );
+
+    let second_items = second.items.as_ref().expect("items must be present");
+    let twin_items2: Vec<_> = second_items
+        .iter()
+        .filter(|i| i.doc_path.0.ends_with("__init__.py"))
+        .collect();
+    assert_eq!(
+        twin_items2.len(),
+        2,
+        "second ingest: expected exactly 2 __init__.py items; items={second_items:?}"
+    );
+    for item in &twin_items2 {
+        assert_eq!(
+            item.kind,
+            IngestItemKind::Unchanged,
+            "second ingest: each twin must be Unchanged; item={item:?}"
+        );
+    }
+}