fix(metrics): simplify macOS memory to use Node.js os module

[Claude]

The vm_stat-based macOS memory calculation was inaccurate. Simplified to use Node.js native os.freemem() and os.totalmem(), matching the Windows approach.

Changes:

• Replaced ~40 lines of vm_stat parsing with 5-line implementation using Node.js os module
• macOS and Windows now share identical memory collection logic
• Linux continues using /proc/meminfo for MemAvailable/Active fields

Before (vm_stat parsing):

// Parse vm_stat output, extract page size, count pages by type
// Calculate: active = (pagesActive + pagesWiredDown) * pageSize
// Calculate: available = (pagesFree + pagesInactive + ...) * pageSize

After (Node.js native):

const available = freemem();
const total = totalmem();
const active = total - available;

To fix this, replace the use of execSync("cat /proc/meminfo", ...) with a direct file read via Node’s fs.readFileSync. This removes the need to spawn a shell and execute cat, while preserving the current behavior of returning the file contents as a UTF‑8 string.

Concretely, in src/system-info.ts, within getLinuxMemory(), change the line:

const meminfo = execSync("cat /proc/meminfo", { encoding: "utf-8" });

to:

const meminfo = readFileSync("/proc/meminfo", { encoding: "utf-8" });

You then need to import readFileSync from the Node fs module at the top of the file. No other logic needs to change; the parsing of meminfo as a string remains the same and the function signature and return type are unaffected.