refactor: split gateway into a Cargo workspace member

[jaredLunde]

What

Moves the single beyond-ai crate into crates/gateway/ and turns the repo root into a virtual Cargo workspace, so additional open-source AI components (an agent harness, etc.) can live beside the gateway without entangling its deps/build.

This is M0 of a larger effort — the structural "make room for crates" step. No agent code yet.

Behavior-neutral

Package beyond-ai and lib beyond_ai are unchanged, so every use beyond_ai::… and CARGO_BIN_EXE_beyond-ai keeps working. The diff is almost entirely git mv renames plus four small manifest/doc edits.

Changes

• root Cargo.toml → [workspace]: members = ["crates/*"], resolver = "3", lints hoisted to [workspace.lints] (so the unsafe/panic-surface denies apply to every future crate root), [profile.release] at root, and [workspace.dependencies] for deps shared with future crates.
• crates/gateway/Cargo.toml: inherits workspace lints/edition/shared deps; gateway-specific deps (pingora, store, rustls…) stay local.
• Dockerfile: build line → cargo build --release -p beyond-ai --bin beyond-ai.
• mise.toml: example task → cargo run -p beyond-ai --example mint_dev_key.
• README: quick-start paths point at crates/gateway/.
• CI/publish workflows: no edits needed — they go through mise tasks + the Dockerfile.

Verification

cargo build · clippy --all-targets -D warnings · cargo fmt --all --check · dprint check · 99 unit · 21 integration (+10 smoke ignored) · benches compile · docker image builds and --help runs.

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