implement builtin.files — workspace file tree with live watcher

Flutter sidebar tab that lazy-loads the workspace tree via IPC
files.ls and refreshes subtrees on files.changed events. Click-to-
open routes through a future editor.open command; until Tier 2
registers it, the execute call no-ops gracefully.

Daemon side adds a new files subsystem:
  - files.root returns the resolved workspace root (git root if
    present, otherwise cwd)
  - files.ls lists a directory with ignore filtering applied
  - files.watch starts a recursive Directory.watch and fans
    FileSystemEvents out as files.changed IPC events
  - FilesService owns the watcher + ignore set lifecycle

IgnoreSet + IgnorePattern implement the common gitignore subset:
anchored (/foo), directory-only (foo/), negation (!foo), **
crossing dirs, ** at trailing position. Built-in layer hides clide-
owned dirs (.git, .pql, .clide, .dart_tool, build, node_modules);
.gitignore + .clideignore at the root layer on top per D-004. Full
multi-file ignore_files: layering from .pql/config.yaml is future
work.

11 new ignore-matcher tests + 5 files.* dispatcher tests.

Co-Authored-By: Claude <noreply@anthropic.com>

app/lib/kernel/src/i18n/catalog/builtin.files_en_us.json

@@ -0,0 +1,5 @@
+{
+  "tab.title": { "translation": "Files" },
+  "loading": { "translation": "Loading…" },
+  "empty": { "translation": "No visible files" }
+}