Replace the pgclirc ConfigObj parser with configparser

[jackwalkerlabs]

Closes #1634. This is the separate follow-up requested in the review of #1631.

The main pgclirc reader now uses stdlib configparser, with an adapter for the existing typed settings and named-query persistence interface. The compatibility findings and intentional restrictions are documented in docs/config-compatibility.rst.

The main differences from using configparser directly are:

Existing Pgcli/ConfigObj behavior	Handling in this change
Case-sensitive names, literal %, ordinary [DEFAULT] section	Preserve them explicitly; disable interpolation and default-section inheritance.
Outer quotes, comma-separated lists and typed accessors	Parse existing syntax and retain the accessors used by Pgcli.
Triple-quoted named queries	Preserve their exact contents, including blank lines, comment-like lines and indentation.
Comments during saves	Update the original file's value spans and retain surrounding comments.
ConfigObj root options and nested [[sections]]	Intentionally unsupported, with explicit errors. Single-bracket dotted sections remain supported.

ConfigObj remains a dependency because the PostgreSQL service-file parser still uses it on the current base branch. This PR leaves that separate migration and #1631 unchanged.

Validation: 52 selected configuration/startup/DSN/named-query tests and all 16 additional compatibility characterizations passed in Factory's clean, offline Python 3.11 verification container, with Ruff lint/format checks. The compatibility characterizations were also run successfully against unchanged upstream code before becoming required checks. The full live-PostgreSQL integration suite and other supported Python versions were not run in this environment.

Prepared and verified through FactoryChief, run FC-20260911-5DE277. The run history includes the failed candidates and operator-guided repairs.