feat(hash-object): add blob hashing command

[marshawcoco]

Summary

• Add libra hash-object for Git-compatible blob OID computation from files or stdin
• Support -w/--write, -t/--type blob, --json, and --machine
• Document the command and remove hash-object from README pending commands
• Add integration coverage for file hashing, stdin hashing, object writes, JSON output, and unsupported object types

Tests

• cargo check
• cargo +nightly fmt --all --check
• cargo test --test command_test hash_object_test -- --nocapture
• cargo clippy --all-targets --all-features -- -D warnings

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 01c9c241af

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 1122a03fbb

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".