chore(v→zig): delete legacy V-lang API shim (#28)

## Summary

Estate-wide V-lang→Zig remediation per the 2026-05-28 owner directive.
Deletes `api/v/conflow.v` (a thin V-lang wrapper that called into the
already-existing Zig FFI at `ffi/zig/`). With V removed from the estate,
the Zig FFI is the canonical API surface; no replacement needed.

## Test plan

- [ ] Hypatia rule \`cicd_rules/vlang_detected\` (in the parallel
hypatia PR) does not flag this repo post-merge

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath