build(deps): remove dependencies nothing references

[justin13888]

Summary

• Cargo.toml: removes shell-words, etcetera and owo-colors from [dependencies], the insta and predicates dev-dependencies, and clap's cargo feature, together with the comments that described the removed crates. None of them is referenced under src/, tests/, build.rs or bench/. clap_complete stays, since CLAUDE.md commits to it for completions.
• Cargo.lock: refreshed by cargo. It drops etcetera, float-cmp, insta, normalize-line-endings, regex, shell-words and the similar 2.7.0 that only insta used. owo-colors and predicates stay in the lockfile only as transitive dependencies, of color-eyre and assert_cmd respectively.
• CLAUDE.md (AGENTS.md is a symlink to it, so both change): the packages line "anstyle / owo-colors" now reads "anstyle". Colour comes from anstyle (src/plan/diff.rs).

Validation

• cargo fmt --check: pass
• cargo clippy --all-targets --locked -- -D warnings: pass
• cargo test --locked: pass (2040 unit tests plus every integration suite; 0 failed)
• pre-commit hooks (line-check, convco): pass

Risks and rollout

• The shipped binary's dependency set gets smaller. The crate is publish = false and ships only as the install.sh binary, so no downstream crate is affected. bx --version is built by build.rs, not clap's cargo macros, and still compiles. To reverse, put the removed lines back in Cargo.toml.
• Coverage gap: none. No behaviour changes, so there is nothing new for a test to reach.

Issue

Closes #145

Decisions taken

• Issue Remove dependencies nothing references #145 records: "None beyond the surface answer below." The surface answer is the maintainer's "Remove all but clap_complete (Recommended)", and this change carries it out.
• Fork: whether to edit AGENTS.md separately from CLAUDE.md. Taken: edit only CLAUDE.md. Rejected: a separate edit, because AGENTS.md is a symlink to CLAUDE.md (ls -l AGENTS.md), so it already shows the change. Reverse: nothing to reverse.