Default local teardown to the default instance

[sdairs]

Summary

• default omitted names to default for ClickHouse server stop and server remove
• apply the same default to Postgres postgres stop and postgres remove
• add Clap parse coverage for omitted and explicit names
• document the bare default-instance lifecycle commands

Why

start creates or resumes the default instance when no name is supplied, but the matching teardown commands required a positional name. This made the natural default-instance lifecycle fail at argument parsing before the command could run.

Impact

Users can now start, stop, and remove the default ClickHouse or Postgres instance without typing its name. Explicit names and existing disambiguation flags continue to work unchanged.

Stack

This PR is stacked on #354 and should be reviewed as the delta from codex/issue-328-install-output-polish.

Closes #326.

Validation

• cargo fmt --all --check
• cargo build -p clickhousectl
• cargo test -p clickhousectl
• cargo clippy -p clickhousectl --all-targets -- -D warnings

[Copilot]

Pull request overview

Makes local teardown commands symmetric with start by defaulting the omitted instance name to "default" for both ClickHouse and Postgres, improving the default-instance lifecycle and reducing clap parse failures.

Changes:

• Default positional <NAME> to "default" for local server stop/remove and local postgres stop/remove.
• Add clap parse tests covering omitted-name defaults and explicit-name preservation.
• Update README examples and wording to document the default-instance teardown flow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
README.md	Documents stop/remove without a name as operating on "default" for ClickHouse and Postgres.
crates/clickhousectl/src/local/cli.rs	Implements clap defaults for teardown commands and adds parse-coverage unit tests.

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