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.

apps/env-test/env-test.S

@@ -0,0 +1,123 @@
+#
+# env-test: Tests environment variable syscalls.
+#
+# 1. Gets CWD (should be "/")
+# 2. Sets TEST=hello
+# 3. Gets TEST (should be "hello")
+# 4. Forks - child gets TEST, verifies it was copied
+# 5. Both print results and exit
+#
+
+.section .text
+.global _start
+
+.equ SYS_EXIT,    0
+.equ SYS_WRITE,   1
+.equ SYS_FORK,    3
+.equ SYS_WAITPID, 6
+.equ SYS_GETENV,  8
+.equ SYS_SETENV,  9
+
+# Helper: write a string (addr in %ecx, len in %edx) to stdout
+.macro WRITE_STR addr, len
+    movl $SYS_WRITE, %eax
+    movl $1, %ebx
+    movl \addr, %ecx
+    movl \len, %edx
+    int $0x80
+.endm
+
+_start:
+    # 1. Get CWD env var
+    movl $SYS_GETENV, %eax
+    movl $key_cwd, %ebx
+    movl $buf, %ecx
+    movl $128, %edx
+    int $0x80
+
+    # Print "CWD="
+    WRITE_STR $label_cwd, $4
+    # Print the value (length returned in eax)
+    movl %eax, %edx         # length
+    movl $SYS_WRITE, %eax
+    movl $1, %ebx
+    movl $buf, %ecx
+    int $0x80
+    WRITE_STR $newline, $1
+
+    # 2. Set TEST=hello
+    movl $SYS_SETENV, %eax
+    movl $key_test, %ebx
+    movl $val_hello, %ecx
+    int $0x80
+
+    # 3. Get TEST
+    movl $SYS_GETENV, %eax
+    movl $key_test, %ebx
+    movl $buf, %ecx
+    movl $128, %edx
+    int $0x80
+
+    # Print "TEST="
+    WRITE_STR $label_test, $5
+    movl %eax, %edx
+    movl $SYS_WRITE, %eax
+    movl $1, %ebx
+    movl $buf, %ecx
+    int $0x80
+    WRITE_STR $newline, $1
+
+    # 4. Fork
+    movl $SYS_FORK, %eax
+    int $0x80
+    testl %eax, %eax
+    jz .child
+
+.parent:
+    pushl %eax              # save child PID
+    WRITE_STR $msg_parent, $15
+
+    # Wait for child
+    popl %ebx
+    movl $SYS_WAITPID, %eax
+    int $0x80
+
+    # Exit
+    movl $SYS_EXIT, %eax
+    movl $0, %ebx
+    int $0x80
+
+.child:
+    # Child: get TEST to verify it was inherited
+    movl $SYS_GETENV, %eax
+    movl $key_test, %ebx
+    movl $buf, %ecx
+    movl $128, %edx
+    int $0x80
+
+    # Print "Child TEST="
+    WRITE_STR $label_child_test, $11
+    movl %eax, %edx
+    movl $SYS_WRITE, %eax
+    movl $1, %ebx
+    movl $buf, %ecx
+    int $0x80
+    WRITE_STR $newline, $1
+
+    # Exit
+    movl $SYS_EXIT, %eax
+    movl $0, %ebx
+    int $0x80
+
+.section .rodata
+key_cwd:          .asciz "CWD"
+key_test:         .asciz "TEST"
+val_hello:        .asciz "hello"
+label_cwd:        .ascii "CWD="
+label_test:       .ascii "TEST="
+label_child_test: .ascii "Child TEST="
+msg_parent:       .ascii "Parent done!\n\0\0"
+newline:          .ascii "\n"
+
+.section .bss
+buf: .space 128