fix(loop): replace catch unreachable with try in self_hosting_loop.zig init (#159)

Replace unsafe `catch unreachable` with `try` for ArrayList initialization
in SelfHostingSession.init(). Allocation can fail, so error propagation is
required instead of asserting impossibility.

Closes #159

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Trinity Agent

src/tri/self_hosting_loop.zig

@@ -196,8 +196,8 @@ pub const SelfHostingSession = struct {
             .safeguards = safeguards,
             .session_id = session_id,
             .start_time = start_time,
-            .target_files = std.ArrayList([]const u8).initCapacity(allocator, 0) catch unreachable,
-            .patch_history = std.ArrayList(SelfPatch).initCapacity(allocator, 0) catch unreachable,
+            .target_files = try std.ArrayList([]const u8).initCapacity(allocator, 0),
+            .patch_history = try std.ArrayList(SelfPatch).initCapacity(allocator, 0),
             .metrics = ImprovementMetrics{},
             .current_branch = current_branch,
             .backup_dir = backup_dir,