Build without the unpublished :inference dependency; support Elixir 1.20

[BobbieBarker]

Summary

The published package can't be built by an external consumer. mix.exs declares {:inference, path: "../inference/apps/inference"}, a local path dependency that only exists in the author's workspace, so mix deps.get / mix compile abort at dependency resolution. The Hex artifact silently drops the path dep, which leaves GEPA.LLM.Adapters.ReqLLM and GEPA.LLM.Adapters.AgentSessionManager referencing an undefined Inference.* namespace.

This PR makes the package buildable and usable with any LLM, and brings it to Elixir 1.20.

Decoupling the optimizer core from :inference

The optimizer core (engine, proposers, Pareto selection, state, strategies, stop conditions) is already provider-agnostic: it reaches a model only through the GEPA.LLM facade, which accepts a plain function, a %GEPA.LLM.Client{}, or GEPA.LLM.Mock. Only the two provider adapters depended on :inference.

• Removed the :inference path dependency and the now-unused agent_session_manager dependency.
• Marked req_llm and req as optional: true; they're used only by the embeddings and RAG/Qdrant adapters, so a consumer that injects its own LLM takes on neither.
• Deleted the :inference-coupled provider adapters (GEPA.LLM.Adapters.ReqLLM, GEPA.LLM.Adapters.AgentSessionManager, and the GEPA.LLM.ReqLLM compatibility shim that delegated to them). Their compile-time struct references (%Inference.Response{} and friends) cannot resolve without the package. The facade's provider constructors (new/2, req_llm/2, agent/2) now raise with guidance to inject an LLM, keeping their arities.

If keeping the ReqLLM/ASM adapters is preferred, the natural home for them is a small companion package that depends on :inference (or its published pieces), so the core stays installable on its own.

Elixir 1.20

• Raised the supported version to ~> 1.20 and moved preferred_cli_env into def cli.
• Fixed issues surfaced by the 1.20 type checker:
  • adapters/mcp.ex: a {tool, args} clause shadowed the following {:error, reason} clause, so error tuples were being rewritten into {:ok, "error", ...} tool selections instead of propagating. Reordered so errors pass through.
  • optimize_anything.ex: removed a dead || 1 fallback on a value that is always truthy.
  • evaluation_batch.ex: removed two clauses that the validation flow makes unreachable.

Tests and metadata

• Updated the GEPA.LLM facade tests for the new Mock-or-raise default contract, and switched the Basic adapter's "normalized client" test to a plain-function LLM. Removed the provider test suites that covered the deleted adapters.
• Strengthened two tautological != nil assertions the type checker flagged.
• Gave the subprocess code-execution test timeout headroom so it does not race cold BEAM boot on loaded machines.
• Dropped the deleted modules from the ExDoc module groups, removed the stale gepa/ entries from the package files, and corrected the maintainers field.

Verification

On Elixir 1.20.1 / OTP 28:

• mix compile --warnings-as-errors — clean
• mix test — 860 pass (844 tests, 16 properties)
• mix credo --strict — no issues
• mix format --check-formatted — clean
• mix dialyzer — passed