feat(llm): add env vars for low-VRAM GPU configuration

[NguyenQS504092s]

Summary

• Add QMD_RERANK_CONTEXT_SIZE env var (default: 2048) for tuning rerank context window
• Add QMD_EMBED_CONTEXT_SIZE env var (default: auto) for tuning embedding context window
• Add QMD_MAX_PARALLELISM env var (default: auto) to cap parallel contexts
• Add QMD_EMBED_BATCH_SIZE env var (default: 32) for embed loop batch size

Follows the per-setting env var + config pattern established in #313.

Problem

On GPUs with ≤4GB VRAM (RTX 3050, GTX 960M, etc.), qmd query, qmd vsearch, and qmd embed crash with OOM errors. The default settings require ~7GB peak VRAM.

Solution

Add granular env vars so users can tune VRAM usage without modifying source:

# Example for 4GB GPU:
export QMD_RERANK_CONTEXT_SIZE=1024
export QMD_EMBED_CONTEXT_SIZE=1024
export QMD_MAX_PARALLELISM=2
export QMD_EMBED_BATCH_SIZE=8
export QMD_EXPAND_CONTEXT_SIZE=1024  # already exists

Each new env var follows the same resolver pattern as QMD_EXPAND_CONTEXT_SIZE from #313:

• Config value takes precedence over env var
• Invalid values print a warning to stderr and fall back to defaults
• Input validation rejects non-positive integers

Changes

src/llm.ts

• Add resolveRerankContextSize(), resolveEmbedContextSize(), resolveMaxParallelism() resolver functions
• Add rerankContextSize and embedContextSize fields to LlamaCppConfig type
• Replace static RERANK_CONTEXT_SIZE with instance field resolved from config/env
• Pass contextSize to createEmbeddingContext() when QMD_EMBED_CONTEXT_SIZE is set
• Cap computeParallelism() result when QMD_MAX_PARALLELISM is set

src/qmd.ts

• Make BATCH_SIZE configurable via QMD_EMBED_BATCH_SIZE env var

README.md

• Document all env vars in the Environment Variables table
• Add "Low-VRAM GPU Configuration" section with example values

Tested on RTX 3050 Laptop (4GB VRAM, Vulkan)

Metric	Default (crash)	With env vars
qmd embed -f (107 docs)	❌ OOM	✅ 2m19s
qmd query	❌ timeout/OOM	✅ ~60s
qmd vsearch	❌ OOM	✅ works
Peak VRAM	~7GB	~3GB

Search quality impact: ~1-2% (rare edge-case truncation).

Test plan

• TypeScript compiles cleanly (tsc --noEmit)
• Tested on RTX 3050 4GB with all env vars set
• Verify default behavior unchanged when no env vars are set
• Verify invalid env var values produce warnings and use defaults

Fixes #329
Relates to #275, #303