Add privacy-preserving search retrieval receipts

[caioribeiroclw-pixel]

Summary

Adds a compact privacy-preserving retrieval receipt to search_code results.

The receipt gives clients a stable receipt_id they can attach to downstream tool spans/evals without exposing query text, file paths, or code content. It includes:

• index snapshot metadata: status, indexed file/chunk counts, index status, last updated, age
• hashes for query, requested path, effective indexed codebase path, extension filter
• result count/requested limit
• per-result rank, score bucket, location hash, chunk hash, and dedupe key

This follows the shape suggested in #382: keep it as a retrieval receipt rather than full agent telemetry.

Validation

From packages/mcp:

../../node_modules/.bin/tsc --noEmit
node --import tsx --test src/*.test.ts

Result: 5/5 MCP tests passed.

Note: the normal pnpm --filter @zilliz/claude-context-mcp test path attempted an install and was blocked by the workspace's ignored-build-script policy, so I ran the same MCP tests directly with tsx after building the workspace core package locally.