fix(embed): skip oversized chunks instead of crashing

[JasonOA888]

Problem

qmd embed crashes when the first chunk exceeds the embedding model context window (2048 tokens for EmbeddingGemma-300M):

Error: Failed to get embedding dimensions from first chunk

On Apple Silicon, this can also trigger a GGML Metal cleanup assertion resulting in SIGABRT.

Solution

Instead of failing on the first chunk, try embedding chunks sequentially until one succeeds:

1. Iterate through chunks attempting to embed each one
2. Log warning for chunks that fail (oversized, context window exceeded)
3. Use first successful embedding to determine vector dimensions
4. Only throw error if ALL chunks fail

Behavior Changes

• Before: Crash if first chunk exceeds context window
• After: Skip oversized chunks with warning, continue with valid chunks

Fixes #303