feat: surface config warnings (hostname mismatch, source errors)

[clavery]

Summary

Config warnings are now surfaced to users instead of being silently swallowed at trace level:

• Hostname mismatch: When CLI --server flag differs from dw.json hostname, users see a warning explaining that config file values are being ignored; this applies to all config sources that may supply server values along with credentials
• Source errors: When a config source throws (e.g., malformed dw.json), users see a warning about the parse failure

Previously these warnings were only visible with trace-level logging. Now users can identify configuration problems or "gotchas" without blocking execution.

Thanks to @SGD1953 for the idea!

Changes

• SDK sources: Re-throw errors after logging at trace level (dw-json.ts, mobify-source.ts, package-json-source.ts)
• SDK resolver: Catch source errors and create SOURCE_ERROR warnings
• CLI config: Log all config warnings at warn level (not trace) - this surfaces both HOSTNAME_MISMATCH and SOURCE_ERROR
• Docs: Added error handling section to extending.md

Test plan

• Unit tests added for SOURCE_ERROR warning behavior
• Existing tests updated to reflect new behavior
• Manual test: Create malformed dw.json and run b2c setup config - should see warning about parse failure
• Manual test: Run with hostname mismatch (b2c code deploy --server different.host.com with different hostname in dw.json) - should see warning about hostname mismatch

closes #84