fix(cargo): symlink tools to rustup binary instead of mise install dir

The mise rust install dir contains relative `cargo -> rustup` symlinks
that break when rustup isn't in the same directory. Symlink directly to
`$CARGO_HOME/bin/rustup` (the real multicall binary) instead.

Move implementation rationale from shell comments to AGENTS.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pythoninthegrass

AGENTS.md

@@ -70,6 +70,10 @@ Database: `mt.db`, Logs: see [Observability](docs/observability.md)
 
 See [Tauri Architecture](docs/tauri-architecture.md) for full details.
 
+### Rust Toolchain (mise + rustup)
+
+mise manages the Rust version (via `.tool-versions`) but delegates to rustup under the hood. The mise install directory (`~/.local/share/mise/installs/rust/<version>/`) contains symlinks like `cargo -> rustup` — these are rustup multicall symlinks that require the `rustup` binary to be resolvable. The `cargo:_setup-cargo-home` task in `taskfiles/cargo.yml` creates `$CARGO_HOME/bin` symlinks pointing directly at `$CARGO_HOME/bin/rustup` (the real binary) rather than chaining through the mise install dir, which would break if the relative `rustup` target is missing.
+
 ## Queue and Shuffle Behavior
 
 The queue store (`app/frontend/js/stores/queue.js`) maintains tracks in **play order** — the `items` array always reflects the order tracks will be played.

taskfiles/cargo.yml

@@ -9,11 +9,15 @@ tasks:
     desc: "Create CARGO_HOME/bin symlinks so mise shims resolve correctly"
     cmds:
       - |
-        rust_dir="$(mise where rust)"
         cargo_bin="$CARGO_HOME/bin"
         mkdir -p "$cargo_bin"
+        rustup_bin="$cargo_bin/rustup"
+        if [[ ! -x "$rustup_bin" ]]; then
+          echo "error: rustup not found at $rustup_bin" >&2
+          exit 1
+        fi
         for tool in cargo cargo-clippy cargo-fmt rustc rustdoc rustfmt; do
-          ln -sf "$rust_dir/$tool" "$cargo_bin/$tool"
+          ln -sf "$rustup_bin" "$cargo_bin/$tool"
         done
     status:
       - test -L "$CARGO_HOME/bin/cargo" && test -e "$CARGO_HOME/bin/cargo"