Phase 1 — Kernel Bring-up: boot to interactive serial shell (v0.2)

[MASTER_PLAN.md §5 Phase 1]. Entry: Phase 0 green.

Deliverables: GDT/IDT/paging/PMM/heap/scheduler init; initramfs loading; syscall path; serial shell; home build (A5).

Exit gate — CI job boot-to-shell:

• Kernel initializes GDT, IDT, paging, PMM, heap, scheduler, loads initramfs
• Userspace hello binary runs via real write/exit syscalls
• Interactive serial shell executes 5 scripted commands (echo, ls, cat, ps, uname) under expect-style harness
• Timer + keyboard IRQ counters exposed via shell and asserted

Blocked by #21 (Phase 0).