logger: write warnings to stderr, not stdout

[selimdev00]

Problem

logger.warn writes to stdout via console.log:

export function warn(message: string): void {
  if (quiet) return;
  console.log(chalk.yellow(message));
}

Warnings are diagnostics, not results, so they belong on stderr. Emitting them on stdout corrupts any machine-readable stdout - e.g. a --json payload or a value being piped/captured - by interleaving a yellow warning line into it. Several real warnings flow through here: corrupt bmalph/config.json, corrupt bmalph/state/current-phase.json, malformed _bmad/config.yaml, and the platform-resolution fallback. A consumer parsing stdout JSON can hit a syntax error purely because the project state was slightly off.

Fix

console.error(chalk.yellow(message));

info/debug are unchanged. The quiet gate is unchanged. This is purely a stream correctness fix.

Tests

• Updated logger tests: warn now asserts on stderr and additionally asserts nothing is written to stdout.
• Updated the three suites that captured warnings via a console.log spy (config, state, platform resolve) to spy on console.error - which also documents that those warnings were previously landing on stdout.
• Full unit suite passes (1836).