Layer 2: add command parity harness

[aboimpinto]

Summary

• Add a command registry parity harness for metadata completeness, name/alias lookup, and per-command help topic coverage.
• Pin slash parser argument preservation for representative model-instruction commands.
• Add representative dispatch-surface checks across the future command groups before moving implementations.
• Add command palette parity coverage so every registered command appears exactly once with the expected palette insertion text and help/usage description.

Context

This is Layer 2 of the command refactor breakdown. PR #2851 remains the reference branch, but this PR keeps the mergeable slice test-only so the later group-owned command-file refactor can prove behavior parity before production code moves.

Refs #2791
Refs #2870
Reference #2851
Follows #2871

Validation

• cargo fmt
• cargo test -p codewhale-tui commands::tests::
• cargo test -p codewhale-tui command_palette
• cargo test -p codewhale-tui slash_completion
• cargo test -p codewhale-tui

Paulo Aboim Pinto

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[github-actions]

Thanks @aboimpinto for taking the time to contribute.

This repository is currently observing a maintainer-managed contribution gate in dry-run mode, so this pull request is staying open. When enforcement is enabled, pull requests from contributors who are not listed in .github/APPROVED_CONTRIBUTORS will be closed automatically.

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[gemini-code-assist]

Code Review

This pull request introduces comprehensive unit tests to validate command registry metadata, alias resolution, help topics, argument parsing, and command palette entries. The feedback suggests optimizing test performance by reusing a single App instance across iterations in the help topic test, and simplifying loop variables by pattern matching &alias to avoid double references when iterating over command aliases.

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