chore: remove project-level sccache rustc-wrapper config

Code-review feedback: sccache shouldn't be a per-project build
dependency. Cargo's incremental cache already covers what sccache
offers for a single project, and forcing rustc-wrapper = "sccache"
project-wide means every contributor has to install sccache or
prepend RUSTC_WRAPPER= to bypass the wrapper.

.cargo/config.toml only existed to wire sccache and pin SCCACHE_DIR
to a project-local cache. Removing the file entirely so plain
`cargo build` works without any extra setup. The .cargo directory
is empty after the deletion and removed too. .gitignore's
/.sccache-cache line is harmless cruft and stays — players who
already have a populated .sccache-cache directory keep it ignored.

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

.cargo/config.toml

@@ -1,31 +0,0 @@
-# Project-wide cargo configuration.
-#
-# Routes every rustc invocation through `sccache` so cold rebuilds and
-# fresh checkouts (CI, new dev box, after a `cargo clean`) replay
-# previously-compiled crates from a local on-disk cache rather than
-# recompiling them. Warm incremental builds still go through cargo's
-# own `target/` cache, which dominates locally — sccache buys you the
-# big wins on cold paths.
-#
-# Requires sccache on PATH. Install it once per machine:
-#
-#   Arch     :  pacman -S sccache
-#   macOS    :  brew install sccache
-#   Cargo    :  cargo install sccache --locked
-#
-# Without sccache the build fails with "rustc-wrapper not found". To
-# bypass this config without editing the file, prepend
-# `RUSTC_WRAPPER= ` (empty value) to your cargo command:
-#
-#   RUSTC_WRAPPER= cargo build
-#
-[build]
-rustc-wrapper = "sccache"
-
-# Project-local cache so the shared dev box (or a Docker volume) keeps
-# the artefacts isolated per checkout instead of mixing them in
-# `~/.cache/sccache`. Set with `force = false` so a developer-set
-# `SCCACHE_DIR` in their shell wins — important because the sccache
-# daemon, once started, sticks with whichever directory it saw first.
-[env]
-SCCACHE_DIR = { value = ".sccache-cache", relative = true, force = false }