Add lint groups for standard convention bundles

[quick-ricon]

Problem

Repos that opt into codebase lint currently enumerate every lint rule individually in mise.toml:

[_.codebase]
lint = ["mise-settings", "gum-table", "bats-test-helper", "bats-test-task", "mcr-scope", "or-true", "shellcheck", "caller-pwd-contract", "github-actions"]

That is explicit, but it creates overhead when many maintained-tool repos want the same convention bundle. It also makes README/docs surfaces more tempting to expose long internal rule lists.

Proposal

Add first-class lint groups / presets, for example:

[_.codebase]
lint_groups = ["maintained-tool"]

or:

[_.codebase]
lint = ["@maintained-tool"]

The exact syntax is open, but the behavior should:

• expand to a stable bundle of named lint rules;
• allow adding individual extra rules;
• allow excluding one rule when a repo has a justified exception;
• keep codebase lint "$PWD" output showing the concrete rules that ran;
• make the group/preset discoverable from codebase --help or docs.

Trigger

This came up while reviewing KnickKnackLabs/shimmer#793. The shimmer README should not need to expose or maintain a long list of internal convention lint names just because the repo uses the standard maintained-tool bundle.