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/syscall.c

@@ -9,6 +9,7 @@
 
 #include "syscall.h"
 #include "process.h"
+#include "env.h"
 #include "port_io.h"
 #include "vga.h"
 #include <stddef.h>
@@ -139,6 +140,37 @@ static int32_t sys_exec(registers_t *regs) {
     return -1; /* Not implemented yet */
 }
 
+/**
+ * Handle SYS_GETENV: get an environment variable.
+ * EBX = key pointer, ECX = value buffer pointer, EDX = buffer size.
+ * Returns length of value, or -1 if not found.
+ */
+static int32_t sys_getenv(registers_t *regs) {
+    const char *key = (const char *)regs->ebx;
+    char *buf = (char *)regs->ecx;
+    uint32_t bufsize = regs->edx;
+
+    process_t *cur = process_current();
+    if (!cur) return -1;
+
+    return env_get(&cur->env, key, buf, bufsize);
+}
+
+/**
+ * Handle SYS_SETENV: set an environment variable.
+ * EBX = key pointer, ECX = value pointer (NULL to unset).
+ * Returns 0 on success, -1 on error.
+ */
+static int32_t sys_setenv(registers_t *regs) {
+    const char *key = (const char *)regs->ebx;
+    const char *value = (const char *)regs->ecx;
+
+    process_t *cur = process_current();
+    if (!cur) return -1;
+
+    return env_set(&cur->env, key, value);
+}
+
 /** System call dispatch table. */
 typedef int32_t (*syscall_fn)(registers_t *);
 static syscall_fn syscall_table[NUM_SYSCALLS] = {
@@ -150,6 +182,8 @@ static syscall_fn syscall_table[NUM_SYSCALLS] = {
     [SYS_YIELD]   = sys_yield,
     [SYS_WAITPID] = sys_waitpid,
     [SYS_EXEC]    = sys_exec,
+    [SYS_GETENV]  = sys_getenv,
+    [SYS_SETENV]  = sys_setenv,
 };
 
 void syscall_handler(registers_t *regs) {