feat: node.linkable — first-class link targets for the [[ picker (§8.4)

The picker listed every node, so each task showed up twice (itself + its
same-titled canonical-context doc) plus tag/log noise — and the new
preview made the duplicates look identical. New `Store::list_linkable_nodes`
/ `node.linkable` returns non-tombstoned nodes minus `tag`s and the docs
that are a task's canonical-context or log attachment (you link the task,
not its body). The Telescope picker now sources from it.

Tests: a socket test (5 nodes → 2 linkable: task + standalone doc).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/hephd/tests/rpc_socket.rs

@@ -274,6 +274,28 @@ fn tag_add_list_remove_over_socket() {
     );
 }
 
+#[test]
+fn node_linkable_excludes_context_docs_logs_and_tags() {
+    let (socket, _dir) = spawn_daemon();
+    let mut c = client(&socket);
+
+    // task.create → a task node + a same-titled canonical-context doc.
+    let task = c.call("task.create", json!({ "title": "Fix roof" })).unwrap();
+    let task_id = task["node_id"].as_str().unwrap().to_string();
+    // a standalone doc, a tag node, and a log doc (first append).
+    c.call("node.create", json!({ "kind": "doc", "title": "Notes" })).unwrap();
+    c.call("tag.add", json!({ "node_id": task_id, "tag": "house" })).unwrap();
+    c.call("log.append", json!({ "task_id": task_id, "text": "started" })).unwrap();
+
+    // Only first-class targets: the task and the standalone doc — not the
+    // context doc, the log doc, or the tag (5 nodes total ⇒ 2 linkable).
+    let nodes = c.call("node.linkable", json!({})).unwrap();
+    let arr = nodes.as_array().unwrap();
+    assert_eq!(arr.len(), 2, "expected just the task + standalone doc:\n{arr:#?}");
+    assert!(arr.iter().any(|n| n["title"] == "Fix roof" && n["kind"] == "task"));
+    assert!(arr.iter().any(|n| n["title"] == "Notes" && n["kind"] == "doc"));
+}
+
 #[test]
 fn wikilinks_expand_on_read_and_collapse_on_write_over_socket() {
     let (socket, _dir) = spawn_daemon();