Developer onboarding and build process reliability.

### What happened

From a fresh clone, on a machine whose `rustup default` was 1.94.0:

```
$ git clone github.com/tobi/disktree && cd disktree && make install
cargo xtask bundle
Updating crates.io index
error: rustc 1.94.0 is not supported by the following package:
xtask@0.10.1 requires rustc 1.97
make: *** [install] Error 101
```

Nothing is wrong with `xtask`. `[workspace.package] rust-version = "1.97"`
(`Cargo.toml:8`) is inherited by `xtask/Cargo.toml` via
`rust-version.workspace = true`, and `.cargo/config.toml` aliases
`cargo xtask` to `cargo run --package xtask --`, so `xtask` is the first
package in the build graph and the one cargo names. Cargo rejects the whole
workspace before compiling anything, so the failure arrives with no useful
platform, dependency or file context.

### Why a fresh clone hits it

CI pins 1.97 (`.github/workflows/{ci,release}.yml`), the README says "Rust 1.97
or newer", but nothing provisions that toolchain for people building from
source. There is no `rust-toolchain.toml`, so a stale `rustup default` is a hard
failure rather than a nudge — and `rustup update` is not something the error
message or the README asks for at that point.

### The 1.97 floor looks over-declared

No dependency in `Cargo.lock` sets `rust-version` (checked `gpui-pre`,
`gpui-kit`, `gpui-base`, `gpui-pre-platform`, `gpui-pre-apple`,
`gpui-omarchy`). Building the workspace with the MSRV check bypassed:

| toolchain | `cargo check --workspace --all-targets --...