fix(ci): unblock container build — V 0.5 + Zig 0.15 compat

- delete dead adapter/v/src/deprecation.v + legacy.v (imported but
  never used in main.v; remnants of pre-V-ban cleanup sweep)
- remove their imports from main.v
- stapeln_ffi.zig: discard unused `replicas` param (Zig 0.15 strict)

Verified locally: `v -shared -check adapter/v/src/` exits 0;
`zig build -Doptimize=ReleaseFast` in stapeln-mcp/ffi produces
libstapeln_mcp.so. Unblocks container-publish.yml which has been
failing since v0.4.0.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath