feat: implement per-process environment variables (AI)

- Created env.c/h: key=value store with ENV_MAX_VARS=32 entries per
  process. Supports get, set, unset, and copy operations.
- Added env_block_t and cwd[128] fields to process_t struct.
- process_create() initializes CWD=/ by default.
- Fork inherits environment via memcpy of the entire process_t.
- Added SYS_GETENV (8) and SYS_SETENV (9) system calls.
- Created env-test user app that verifies: CWD default, set/get,
  and inheritance across fork.
- Updated kernel.c to launch 'sh' as init process (for next task).
- Updated README.md to check off environment variables task.

Tested: env-test reads CWD=/, sets TEST=hello, reads it back,
forks child which inherits TEST=hello. All verified via QEMU.

src/CMakeLists.txt

@@ -19,6 +19,7 @@ add_executable(kernel
     cpio.c
     vfs.c
     initrd_fs.c
+    env.c
     interrupts.S
     kernel.c
 )