Keep stopped ClickHouse servers discoverable

[sdairs]

Summary

• retain ClickHouse server identity metadata with pid: 0 after clean stops, foreground exits, failed starts, and stale-process detection
• clear and omit transient ClickHouse version and port details while stopped because each start resolves them again
• reject client and dotenv connections to stopped entries
• add subprocess coverage for start/stop/list, stale metadata normalization, and stopped connection commands
• document retained identity metadata and stopped-server listing

Stack

This PR is stacked on #355 and should be reviewed as the delta from codex/issue-326-default-local-instance.

Closes #324.

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

Retains ClickHouse local-server metadata after a server stops so stopped instances remain discoverable in clickhousectl local server list, while clearing transient runtime details (PID/version/ports) for stopped ClickHouse entries and preventing connection helpers from using stopped metadata.

Changes:

• Preserve ClickHouse server metadata on stop/foreground-exit/failed-start and normalize stale PID metadata to a stopped sentinel (pid: 0, cleared ports/version).
• Hide ClickHouse version/ports in list output when stopped, and reject local client / local server dotenv lookups against stopped entries.
• Add subprocess regression tests covering stop/list normalization and stopped-connection failures; update README docs accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Documents retained identity metadata for stopped servers and listing behavior.
crates/clickhousectl/tests/local_server_stopped_test.rs	Adds subprocess regression coverage for stopped metadata retention/normalization and connection rejection.
crates/clickhousectl/src/local/server.rs	Introduces mark_server_stopped, keeps metadata on stop, and normalizes stale ClickHouse PIDs to “stopped”.
crates/clickhousectl/src/local/mod.rs	Rejects client/dotenv commands for stopped entries; suppresses ClickHouse runtime fields in list output when stopped; keeps metadata after foreground exit.
crates/clickhousectl/src/local/cli.rs	Updates CLI help text for list/stop/stop-all to reflect preserved metadata behavior.

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[iskakaushik]

LGTM