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>

lib/clide.dart

@@ -9,7 +9,11 @@
 library;
 
 export 'src/daemon/dispatcher.dart';
+export 'src/daemon/files_commands.dart';
 export 'src/daemon/pane_commands.dart';
+export 'src/files/ignore.dart';
+export 'src/files/listing.dart' show FileEntry, listDir;
+export 'src/files/watcher.dart';
 export 'src/ipc/envelope.dart';
 export 'src/ipc/paths.dart';
 export 'src/ipc/schema_v1.dart';