refactor: move shared helpers to their correct files

[nathanjcochran]

Follow-up to #179, addressing review feedback about where shared helpers should live.

The main outcome is a written-down rule for helper placement in CLAUDE.md — place a helper by who calls it, working down the list until one matches:

1. One command → that command's file.
2. Several commands in one group → the group file.
3. Across groups → a named package-level file.
4. A genuine standalone utility, with no notion of a command → internal/util.
5. Used by both CLI and MCP → internal/common.

Applying it:

• New terminal.go for checkStdinIsTTY, readPasswordFromTerminal, and readString.
• New password.go for the password rotation helpers shared by db connect and service update-password.
• All shell completion functions now live in completion.go (serviceIDCompletion, configOptionCompletion, mcpGetCompletion), matching how ghost does it.
• generateSecurePassword moved to internal/util as util.GenerateSecurePassword.
• read_replica.go and password_recovery.go folded into db_connect.go — both were reachable only from db connect.
• The service list output chain moved out of service.go into service_list.go, and MCP's ServiceInfo/convertToServiceInfo out of utils.go into service_list.go.

Also fixed two stale .goreleaser.yml references in CLAUDE.md.

As with #179 this is a pure reorganization — every moved declaration is byte-identical to before, verified by diffing declaration text against main. The exceptions are exporting GenerateSecurePassword plus its two call sites, and splitting one var block so the terminal helpers could move independently.

[Askir]

lgtm! Thank you!

[Askir]

any opinion about giving files like this one that do not contain any actual cli command a different name so they are more easily identifiable (like the _test for the test files)?

E.g. password_util.go or password_helper.go?