feat(graphile-settings): enable many-to-many relations by default

[pyramation]

Summary

Flips the many-to-many preset from opt-in (@behavior +manyToMany required per junction table) to enabled by default (upstream @graphile-contrib/pg-many-to-many default behavior).

The ManyToManyOptInPlugin previously injected -manyToMany into the inferred behavior phase, suppressing m2m fields unless explicitly opted in. That entityBehavior override is now removed, making the plugin a no-op passthrough. The upstream plugin's default (m2m enabled) takes effect.

Combined with the existing _manyToManyRelation inflector in custom-inflector.ts, this produces clean relation names like event.contacts instead of event.contactEventsByEventId, with automatic fallback to verbose names on conflict.

Export names (ManyToManyOptInPlugin, ManyToManyOptInPreset) are preserved for backwards compatibility.

Review & Testing Checklist for Human

• Schema snapshot test will break — graphql/server-test/__tests__/schema-snapshot.test.ts uses toMatchSnapshot() and the snapshot was NOT regenerated in this PR. New m2m fields will be added to every junction table in the test schema (post_tags). Run the test with --updateSnapshot and verify the new fields look correct.
• Verify upstream default — this PR assumes @graphile-contrib/pg-many-to-many enables m2m by default when no behavior override is present. Confirm this matches the installed version's actual behavior.
• Breaking change for existing consumers — any project using ConstructivePreset will see new m2m connection fields appear on types with junction tables. Verify this is acceptable for all downstream projects, or consider a migration/versioning strategy.
• Test with a real schema — spin up a PostGraphile server with the ConstructivePreset against a database with junction tables and confirm the generated schema has clean m2m field names (e.g., contacts not contactEventsByEventId).

Notes

• The plugin name ManyToManyOptInPlugin is now semantically misleading since it no longer enforces opt-in. Consider renaming in a follow-up if that causes confusion.
• Opt-out is available per junction table via COMMENT ON TABLE ... IS E'@behavior -manyToMany'.

Link to Devin session: https://app.devin.ai/sessions/5c8c35d8c2f3473dbc021b96fe2768c1
Requested by: @pyramation

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