refactor: replace console.log with structured logger calls

Replace raw `console.log`/`console.error` calls across CLI, API
handlers, and module files with structured logger functions (`step`,
`done`, `warn`, `fail`) from the shared logger library.

This improves log consistency, readability, and makes it easier to
control output formatting and log levels from a single place.

src/shared/lib/logger.js

@@ -0,0 +1,30 @@
+/**
+ * ZEN Console Logger
+ * Centralized, styled logging for server-side output.
+ * Inspired by the zen-start CLI style.
+ */
+
+const c = {
+  reset:  '\x1b[0m',
+  dim:    '\x1b[2m',
+  bold:   '\x1b[1m',
+  green:  '\x1b[32m',
+  yellow: '\x1b[33m',
+  red:    '\x1b[31m',
+  cyan:   '\x1b[36m',
+};
+
+const out = (msg) => process.stdout.write(msg + '\n');
+
+/** In-progress step — dim ◆ */
+export const step  = (msg) => out(`  ${c.dim}◆${c.reset} ${msg}`);
+/** Success — green ✓ */
+export const done  = (msg) => out(`  ${c.green}✓${c.reset} ${msg}`);
+/** Warning — yellow ⚠ */
+export const warn  = (msg) => out(`  ${c.yellow}⚠${c.reset} ${msg}`);
+/** Error / failure — red ✗ */
+export const fail  = (msg) => out(`  ${c.red}✗${c.reset} ${msg}`);
+/** Sub-item detail — dim · */
+export const info  = (msg) => out(`    ${c.dim}·${c.reset} ${msg}`);
+
+export const logger = { step, done, warn, fail, info };