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/heph-core/src/sqlite/mod.rs

@@ -289,6 +289,10 @@ impl Store for LocalStore {
         nodes::list(&self.conn, &self.owner_id, kind)
     }
 
+    fn list_linkable_nodes(&self) -> Result<Vec<Node>> {
+        nodes::list_linkable(&self.conn, &self.owner_id)
+    }
+
     fn journal_open_or_create(&mut self, date: &str) -> Result<Node> {
         let now = self.clock.now_ms();
         nodes::open_or_create_journal(&self.conn, &self.owner_id, now, date)