add search-and-replace across files

Builds on the find-in-files engine. A replace engine applies the
query's replacement to each matching file — literal or regex with
capture-group expansion ($1, $&, $$) — and reports per-file, per-line
before/after edits computed with the same logic the apply uses, so
preview and apply never disagree.

The search.replace command previews (no disk writes) or applies
(writing each changed file through the workspace path-safety guard).
The panel gains a Replace field: each match row previews its rewritten
line, and Replace all is gated on a clean git working tree (git is the
undo) plus a confirmation before it writes.

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

CHANGELOG.md

@@ -18,6 +18,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- Search-and-replace across the workspace: enter a replacement in the search
+  panel to preview each rewritten line, then Replace all (regex capture groups
+  supported). Guarded by a clean-git-tree gate — git is the undo — and a
+  confirmation. New `search.replace` command (preview + apply). (T-53)
 - Find-in-files sidebar panel (Ctrl/Cmd+Shift+F): search the workspace with
   regex and case toggles plus include/exclude globs; results stream in grouped
   by file and clicking a match opens the editor at that line. (T-52)