fix(code-indexing): use sha256 for Milvus IDs

[kilo-code-bot]

Summary

• Replaces MD5 with SHA-256 for deterministic Milvus chunk IDs to resolve CodeQL alert 232.
• Keeps ID generation deterministic and within the existing 64-character Milvus primary key limit.

Verification

N/A; code-only hashing change with no meaningful manual flow to exercise in this environment.

Visual Changes

N/A

Reviewer Notes

• Scope intentionally limited to CodeQL alert 232 in apps/web/src/lib/code-indexing/milvus-storage.ts.
• Existing documents keep their old IDs until files are reindexed; reindexing deletes by file path before upserting new SHA-256 IDs.