fix(completion): complete commands after pipe and other separators

[Fanteria]

Summary

• Treat tokens that follow |, ||, &&, ;, or & as command position in Config::get_completions and get_completions_using_basic_lookup, so they complete from $PATH (commands, builtins, functions, aliases) instead of $PWD files.
• Previously only the token at index 0 was treated as command position, so cat /dev/null | ec<TAB> would offer file matches (e.g. echoing_file) rather than echo.
• Add integration tests covering completion after | and &&, asserting that the builtin appears and a same-prefix file in $PWD does not.

Known limitations

• Coloring of these commands is not addressed in this PR.
• Completion still differs from bash in some argument-position cases. For example, grep ec<TAB>: bash performs no completion (grep's first argument is a pattern with no completion spec), while brush falls back to $PATH completion.

Validation

• cargo test -p brush-shell --test brush-completion-tests native_complete_command_after_pipe
• cargo test -p brush-shell --test brush-completion-tests native_complete_command_after_and_operator
• cargo clippy -p brush-core --all-targets
• cargo +nightly fmt -- $(git diff --name-only HEAD -- '*.rs')

Manual verification

1. Build this branch: cargo build --bin brush.
2. In a directory containing a file like echoing_file, launch ./target/debug/brush.
3. Type cat /dev/null | ec and press <TAB>, confirm it completes to echo and ecpg.
4. Type true && ec and press <TAB>, confirm the same behavior.
5. Type ec at the start of a line and press <TAB> — confirm echo and ecpg still complete.