Emit compile-time deprecation warning for legacy rate-limit frontmatter

[Copilot]

rate-limit is a deprecated alias of user-rate-limit, but compilation only logged this at debug level, so users had no migration signal. This change surfaces a user-facing compiler warning while keeping legacy behavior non-breaking.

• Compiler deprecation signal (user-facing)

  • In extractRateLimitConfig, when the legacy rate-limit key is used, emit a warning to stderr via console.FormatWarningMessage.
  • Warning text explicitly points authors to user-rate-limit.
  • Warning remains non-fatal (compilation still proceeds).

• Warning accounting

  • Increment compiler warning count for legacy-key usage to align with other compile-time warning paths.

• Integration warning coverage

  • Extend rate_limit_experimental_warning_test.go with a legacy rate-limit case asserting:
    • deprecation warning is emitted
    • replacement key (user-rate-limit) is named
    • non-legacy cases do not emit the legacy warning.

if legacyKey {
	fmt.Fprintln(os.Stderr, console.FormatWarningMessage(
		"'rate-limit' is deprecated. Use 'user-rate-limit' instead."))
	c.IncrementWarningCount()
}