feat(helexa-acp): scaffold ACP bridge with provider trait + OpenAI chat

Adds a new workspace crate `helexa-acp` (binary, Apache-2.0) — the
start of "the missing ACP binary" for multi-endpoint LLM setups
mixing public APIs, private LAN deployments, and various wire
formats. Today it speaks OpenAI /v1/chat/completions; the
Provider trait is the seam that lets OpenAI Responses, Anthropic
/v1/messages, and other wire formats slot in later without touching
the agent loop.

The crate is intentionally self-contained — no dependencies on the
other workspace crates (cortex-core, cortex-gateway, neuron) — so a
future migration to a dedicated GitHub repo is a Cargo.toml-only
change. All deps come from crates.io.

This commit lands:

  * `config.rs` — TOML config at $XDG_CONFIG_HOME/helexa-acp/config.toml
    with multi-endpoint support (each `[[endpoints]]` declares its
    name, base_url, wire_api, default_model, optional API key /
    api_key_env). Falls back to env-only single-endpoint config when
    no TOML exists (HELEXA_ACP_BASE_URL, HELEXA_ACP_MODEL, etc.). The
    `endpoint:model` selector syntax is validated and tested.

  * `provider/mod.rs` — `Provider` trait + provider-agnostic types
    (`CompletionRequest`, `CompletionEvent`, `Message`, `ToolCall`,
    `ToolSpec`, `Role`, `UsageStats`). Agent loop consumes these
    without knowing the wire format on the other side.

  * `provider/openai_chat.rs` — `OpenAIChatProvider` impl. Compatible
    with cortex, LM Studio, Ollama (compat mode), OpenRouter, OpenAI
    itself. Streams via reqwest + eventsource-stream + async-stream.
    Surfaces text deltas, reasoning deltas (for models that emit
    `reasoning_content`), tool-call lifecycle (start, args-delta,
    completion), usage, finish reason. Cancellation-token aware.

  * `main.rs` — tokio + stderr-only tracing-subscriber + Stdio
    transport. Builds a provider per configured endpoint at startup,
    surfacing config mistakes before the editor even initializes.
    Currently responds to `initialize`; everything else stubs to
    `not implemented yet` until the agent loop lands in the next
    commit.

12 unit tests pass — encoder shape, decoder shape (text-only,
tool-call progressive, cancellation, malformed-chunk recovery),
config parsing (multi-endpoint TOML, env fallback, validation).

The `#![allow(dead_code)]` on `provider/mod.rs` is temporary — the
agent loop in the next commit reads every field. It's noted in the
module-level docstring so the next reader knows.

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

crates/helexa-acp/Cargo.toml

@@ -0,0 +1,39 @@
+[package]
+name = "helexa-acp"
+version = "0.1.16"
+edition = "2024"
+license = "Apache-2.0"
+repository = "https://git.lair.cafe/helexa/cortex"
+description = """
+Agent Client Protocol bridge for the helexa self-hosted LLM stack.
+Speaks ACP to ACP-compatible editor clients (Zed, etc.) and forwards
+the conversation to any OpenAI-compatible HTTP endpoint — defaulting
+to cortex (helexa's reverse-proxy / fleet gateway).
+"""
+
+# This crate is intentionally self-contained — no dependencies on other
+# workspace crates (cortex-core, cortex-gateway, neuron). The goal is
+# a painless migration to a dedicated GitHub repo in the future if the
+# project grows beyond helexa's needs. All deps are crates.io.
+[dependencies]
+agent-client-protocol = "0.12"
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "io-util", "process", "signal"] }
+reqwest = { version = "0.12", features = ["json", "stream", "rustls-tls"], default-features = false }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+toml = "0.8"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+anyhow = "1"
+thiserror = "2"
+async-trait = "0.1"
+futures = "0.3"
+tokio-stream = "0.1"
+tokio-util = { version = "0.7", features = ["rt"] }
+eventsource-stream = "0.2"
+async-stream = "0.3"
+url = { version = "2", features = ["serde"] }
+
+[[bin]]
+name = "helexa-acp"
+path = "src/main.rs"