fix(command-runner): grow command string buffer instead of assuming capacity

[ratazzi]

Problem

executeCommandWithLogging built its display/logging command string with appendAssumeCapacity/appendSliceAssumeCapacity on a buffer allocated with initCapacity(allocator, 256) (falling back to capacity 0 on allocation failure). The AssumeCapacity variants never grow the buffer, so any argv longer than 256 bytes wrote out of bounds.

This triggers on routine paths: apt_bootstrap.zig passes sudo <apt> install -y pkg1 pkg2 ... with the full package list, and the brew bundle path in apply.zig does the same. A dozen packages easily exceeds 256 bytes.

Impact: assertion panic in Debug/ReleaseSafe; heap corruption in ReleaseFast.

Fix

• Use the allocating append/appendSlice variants so the buffer grows as needed.
• Extract the string building into a joinArgs helper and cover it with tests, including an argv well past the initial capacity.
• Add command_runner.zig to the test-discovery comptime block in main.zig (its tests were not being collected before).

Testing

• zig build test — 11 tests pass (2 new).