Make omitted local server selection deliberate

[sdairs]

Summary

• preserve omitted ClickHouse server names through clap and dispatch while retaining positional and compatibility --name forms
• make bare project-scoped server stop prefer an existing default, otherwise select the sole known ClickHouse server, succeed as a no-op with none, and require a name or stop-all with multiple non-default servers
• keep bare server remove conservative: remove only an existing default, never infer a custom server, and return safe available-state guidance
• distinguish implicit selection failures from explicit unknown-name typos in human and structured JSON output without expanding the bounded local error-code taxonomy
• document the stop/remove policy and update the structured-error example

Closes #473

Tests

• isolated subprocess coverage for zero, one, many, default, custom, running, stopped, Postgres-only, positional, and compatibility --name states
• cargo fmt --all --check
• cargo build -p clickhousectl
• cargo check -p clickhousectl --no-default-features
• cargo test -p clickhousectl
• cargo clippy -p clickhousectl --all-targets -- -D warnings

Stack

• Position: child 7 of 7 in gh-stack #489
• Base: Emit structured local runtime errors in JSON mode #510 (issue-475-local-structured-errors)
• Parent PR is unchanged and should merge first.

[cursor]

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