chore(cargo): declare MSRV via rust-version = "1.91"

[YOMXXX]

Summary

`rtk` already uses `str::floor_char_boundary()` (`src/cmds/system/pipe_cmd.rs:140`), stabilized in Rust 1.91.0, but `Cargo.toml` doesn't declare the MSRV. Distribution packagers on older toolchains see a confusing `use of unstable library feature` rustc error instead of a clear cargo-level MSRV diagnostic.

Reproduction

```bash

Without this PR, on Rust 1.88.0:

$ cargo check
error[E0658]: use of unstable library feature `round_char_boundary`
--> src/cmds/system/pipe_cmd.rs:140:21
```

Fix

Add `rust-version = "1.91"` to the `[package]` table.

```toml
[package]
name = "rtk"
version = "0.34.3"
edition = "2021"
rust-version = "1.91" # ← added
```

After

```bash
$ cargo +1.88 check
error: rustc 1.88.0 is not supported by the following packages:
rtk@0.34.3 requires rustc 1.91
```

Cargo gives the right diagnostic at the `[package]`-validation step, before `rustc` ever runs.

Test plan

• `cargo +1.93 fmt --all -- --check` clean.
• `cargo +1.93 clippy --all-targets` zero warnings.
• `cargo +1.93 test --bin rtk -- --test-threads=8` → 1909 passed.
• `cargo +1.88 check` reproduces the friendly MSRV error (intended behaviour).

Notes

@DavidReque expressed interest a month ago but no PR materialized — happy to defer if they'd prefer to push their own; otherwise this lands the minimal one-line change so packagers stop hitting the unhelpful rustc message.

Fixes #1402