R2-8.1: gate qdrant feature in zeph-skills compilation

[bug-ops]

Epic: #282 | Phase: 3 | Priority: P1 | Size: S

Problem

zeph-skills always compiles with qdrant feature (pulls blake3, qdrant-client, serde_json, uuid) even when no Qdrant server is used.

Solution

Gate zeph-skills/qdrant behind a workspace-level feature. Skill matching fallback (keyword-based) already exists.

Files: root Cargo.toml, zeph-core/Cargo.toml, agent/mod.rs (cfg-gate QdrantSkillMatcher).