Add shared embed_fn helper to AnyProvider

[bug-ops]

Part of #391

Problem

Identical embed closure pattern duplicated in src/main.rs for create_skill_matcher (lines 705-709) and create_mcp_registry (lines 1164-1168). Each consumer defines its own EmbedFuture type alias.

Proposed Solution

Add impl AnyProvider { pub fn embed_fn(&self) -> EmbedFn } to zeph-llm.

Priority: P1 | Effort: Small