Unify Rocq CI opam setup

[ejgallego]

Summary

• reuse the active opam switch from ocaml/setup-ocaml instead of creating a separate local Rocq switch
• define Rocq CI dependencies in tests/rocq-ci.opam, install them explicitly with opam install --deps-only, and let setup-ocaml cache the populated switch
• enable the OCaml action's dune cache for the Rocq CI job
• make Rocq command resolution and tests cover project-local _opam, BEAM_ROCQ_CMD, and active-PATH coq-lsp discovery
• keep the fast broker suite Lean-only and document the dedicated Rocq suite

Testing

• lake build beam-cli beam-daemon beam-client beam-daemon-rocq-smoke-test
• bash scripts/lint-shell.sh
• bash tests/test-broker-fast.sh
• Rocq suite not run locally because coq-lsp is not installed in this workspace PATH

Notes

• opam-local-packages informs ocaml/setup-ocaml's local pin automation and cache inputs; it does not replace the explicit opam install ... --deps-only step
• the Rocq runtime now falls back to coq-lsp on PATH after checking project-local _opam roots