feat: add lint groups for standard convention bundles (#69)

[olavostauros]

Summary

Adds first-class lint groups / presets so repos can opt into convention bundles with a single reference instead of enumerating every rule.

Fixes #69.

Syntax

Use @group-name in the existing [_.codebase].lint array:

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

Exclude a rule from a group:

[_.codebase]
lint = ["@maintained-tool"]
lint_exclude = ["caller-pwd-contract"]

Add extra rules beyond the group:

[_.codebase]
lint = ["@maintained-tool", "mise-usage-examples"]

Built-in groups

Group	Rules
@maintained-tool	9 rules: mise-settings, gum-table, bats-test-helper, bats-test-task, mcr-scope, or-true, shellcheck, caller-pwd-contract, github-actions

Files changed

File	Change
lib/lint-groups.sh	New — group registry, expansion functions, members lookup
lib/codebase-config.sh	Source lint-groups.sh, expand @groups in codebase_configured_lint_rules(), support lint_exclude
.mise/tasks/lint/_default	Show expanded group count preamble; updated error hint to mention groups
.mise/tasks/lint/groups	New — lint:groups discoverability task
test/lint/lint-groups/lint-groups.bats	New — 17 BATS tests (library + aggregate dispatch)

Validation

• mise run test — 235/235 tests pass (17 new + 218 existing, zero regressions)
• mise run lint:groups — discoverability task works
• mise run lint:groups --all — shows expanded rules per group
• bash -n on all modified files — clean
• git diff --check — no whitespace errors

Self-hosting

The codebase repo does not use [_.codebase].lint, so self-hosting is trivially clean. A downstream test with a synthetic repo using @maintained-tool passes all 9 rules.