jj-core: add description to Cargo.toml

[logarithmone1128]

Checklist

If applicable:

• I have updated CHANGELOG.md — not user-visible, and jj-core: add LICENSE file #10067 did not add an entry for the sibling LICENSE change either
• I have updated the documentation (README.md, docs/, demos/) — n/a
• I have updated the config schema (cli/src/config-schema.json) — n/a
• I have added/updated tests to cover my changes — n/a for manifest metadata
• I fully understand the code that I am submitting (what it does, how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with an eye towards deleting anything that is irrelevant, clarifying anything that is confusing, and adding details that are relevant. This includes, for example, commit descriptions, PR descriptions, and code comments.

---

jj-core is the only package in the workspace without a description, including testutils and gen-protos, which set publish = false and still have one.

jj-cli                 'Jujutsu - an experimental version control system'
jj-lib                 'Library for Jujutsu - an experimental version control system'
jj-core-proc-macros    'Proc macros for the jj-core crate'
testutils              'Integration test utils for the jj-lib crate'
gen-protos             'Generate Protocol Buffers definitions for the jj-lib crate'
jj-core                None

cargo package -p jj-core warns about it today:

warning: manifest has no description
  |
  = note: see https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata for more info

The warning is gone after this change, and Cargo.lock is untouched. crates.io requires the field, and jj-core is publishable — no publish = false — but not yet on crates.io, so the crate cannot go out as it stands.

The wording follows jj-lib's pattern and reflects what the crate holds: content hashing, DAG walking, diff and merge, matchers, and the path/name/object-id types.

I have not reproduced the crates.io rejection itself, only the local cargo package warning; cargo package still fails further along for an unrelated reason, since jj-core-proc-macros is not on the index yet either.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.