Move Rust crates under crates

[phongndo]

Summary

• Move each Rust workspace crate from root-level hz-* directories into crates/.
• Update workspace member discovery and workspace dependency paths for the new layout.
• Refresh the installer include path and README architecture paths.

Motivation

• Problem: Root-level crate directories clutter the repository root and differ from the expected Rust workspace layout.
• Why now: Keeping all Rust crates under crates/ makes the workspace easier to scan and leaves the root for project-level files.

Changes

• Moved hz-cli, hz-bench, hz-command, hz-core, hz-git, and hz-worktree under crates/.
• Replaced the explicit workspace member list with crates/*.
• Updated workspace dependency paths to point at crates/hz-*.
• Adjusted the hz-cli embedded installer script path for the extra directory level.
• Updated the README architecture section to show the new crate paths.

Verification

• cargo check --workspace --all-targets --all-features --locked
• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-targets --all-features --locked
• cargo build --workspace --all-targets --all-features --locked
• Additional: git diff --cached --find-renames --check
• Not run: benchmark smoke and installer/update smoke; this change only reorganizes crate paths and the full workspace build and tests passed.

Risk

• Blast radius: Cargo workspace discovery, internal path dependencies, and tooling that references crate paths.
• Rollback: Move the crates back to the repository root and restore the previous workspace member/dependency paths and installer include path.

Reviewer Notes

• Review focus: confirm the crate moves are detected as renames and that all path updates account for the extra crates/ directory level.