fix: unify workspace show divider widths

[694410194]

Summary

• use a shared divider width constant in workspace show
• align the main and nested section separators to the same width
• add a small unit test to lock the width expectation

Testing

• cargo fmt --manifest-path Cargo.toml --package cortex-cli
• cargo check --manifest-path Cargo.toml -p cortex-cli
• git diff --check
• cargo test --manifest-path Cargo.toml -p cortex-cli test_workspace_show_rule_width_is_consistent (blocked by a pre-existing private import error in src/cortex-cli/src/agent_cmd/tests.rs:7 for read_file_with_encoding)

/claim #53266

[greptile-apps]

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