merge the pql search panel into the unified Search tab

The pql sidebar panel and the find-in-files tab were duplicate search
surfaces. Consolidate into one Search tab with a mode switch: Find
(content grep), Vault (pql ranked search), Query (PQL DSL), Markdown
(the synced markdown-file listing, keeping focus-highlight + live
refresh). SearchPanelView holds both FindInFilesController and
PqlController; the pql body + result rows move into a reusable
PqlSearchBody. The standalone builtin.pql sidebar tab is removed (one
fewer tab — eases the rail); the pql extension keeps the Backlinks
context panel. No D-79 conflict — grep vs ranked search remain distinct
backends, this is UI consolidation.

Adds the pql builtin's first widget/controller tests (it was untested,
so folding it into the tested Search panel required covering the
Vault/Query/Markdown modes + PqlController).

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

CHANGELOG.md

@@ -45,6 +45,13 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   (back, forward, jump-to-pin) and edit pencil. The ticket reader also joins the
   shared retained nav. (T-189, T-190, T-191, T-198, T-199)
 
+### Changed
+
+- The pql search panel merged into the Search tab, which now has modes: Find
+  (content grep), Vault (pql ranked search), Query (PQL DSL), and Markdown (the
+  synced file listing). The standalone pql sidebar tab is gone; Backlinks stays
+  in the context panel. (T-201)
+
 ### Fixed
 
 - The sidebar icon rail no longer overflows when there are more tabs than fit: