cli: retire s2s and move offline tooling into metis

[meaningforger]

Summary

• Remove the standalone s2s executable and its build targets without a compatibility alias.
• Move offline model, project, and query commands into the metis executable.
• Update current specifications, draft RFCs, examples, smoke checks, and upstream conformance scripts. Keep s2sbench separate.

New commands

• metis model validate|inspect|format
• metis project validate|inspect|diff
• metis query compile

Verification

• make check
• go test ./cmd/metis
• git diff --check