Pin the release toolchain to 1.98

[FAZuH]

Summary

The release cut for the glyph fix failed on toolchain drift: the release
workflow installs stable, which resolved to rustc 1.99.0 on the runner,
and 1.99 changed poise's parent_commands method shape, so
ctx.parent_commands fails to compile at two sites
(src/plugin/command.rs:928, :973).

The PR-checks workflow was pinned to 1.98 in #184 for a different 1.99 issue
(the double_must_use lint). This pins _release.yml the same way, so the
two toolchains cannot drift apart again.

Why my local check passed while CI failed

I reproduced locally with cargo +stable check --release --workspace --all-features --all-targets — clean, because local stable is 1.98.1 and
CI's is 1.99.0. They are different compilers. The tell was in CI's own log:

stable-x86_64-unknown-linux-gnu updated - rustc 1.99.0 (from rustc 1.98.1)

Reading the toolchain version from the failing run's log should have come
before reproducing locally; "stable" does not mean the same thing on both
machines.

Changes

_release.yml: dtolnay/rust-toolchain@stable → @1.98, with a comment
naming the reason and cross-referencing the PR-checks pin.

Notes

• The failed run's Patch Version step succeeded before the build failed, so
  the version may have moved once already. The re-run should reuse it, not
  double-bump.
• The changelog, publish and deploy steps were skipped on the failure; this is
  their first real run.
• This PR must merge without [no ci], per the maintainer's instruction,
  so the release actually re-runs and ships.

[github-actions]

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0.4.1 (2026-10-04)