docs: update GRAPHILE.md with plugin architecture and current state

[pyramation]

docs: update GRAPHILE.md with plugin architecture and current state

Summary

Updates the existing root-level GRAPHILE.md (previously just RC dependency management docs) into a comprehensive reference for the PostGraphile v5 plugin ecosystem. This captures the architectural state after the recent v5 migration work (PRs #797–#815).

New sections added:

• ConstructivePreset composition tree showing how all 14 sub-presets compose
• Active package descriptions for all 11 maintained plugins with import examples
• Legacy directory inventory clarifying which graphile/ directories are npm-installed v4 packages vs maintained source
• Declarative operator factory API explaining how satellite plugins (search, PostGIS, trgm) register custom filter operators
• Key design decisions (condition disabled, unified search, opt-in many-to-many, etc.)
• Testing matrix with commands and coverage areas

Existing content updated:

• RC dependency management section preserved but reorganized under its own heading
• Package list updated to reflect current state (old separate search plugins replaced by unified graphile-search, added graphile-postgis, graphile-misc-plugins, etc.)
• Removed postgraphile-plugin-connection-filter from pinned versions table (replaced by v5-native graphile-connection-filter)

No code changes — documentation only.

Updates since last revision

• Moved documentation from graphile/GRAPHILE.md back to root GRAPHILE.md to update the existing file rather than creating a duplicate
• Merged new plugin architecture content with existing RC dependency management content into a single unified document

Review & Testing Checklist for Human

• Verify import paths are correct. The doc shows import { ConstructivePreset } from 'graphile-settings/presets' — confirm this is the actual public import path used by consumers (the preset source file imports makePgService from postgraphile/adaptors/pg, not from graphile-settings).
• Check filter argument naming consistency. The doc consistently uses where as the filter argument name — verify this matches what the GraphQL schema actually exposes after the recent filter → where rename work.
• Spot-check search field naming conventions. The graphile-search section describes the {column}{Algorithm}{Metric} pattern (e.g. bodyBm25Score, titleTrgmSimilarity) — verify against actual generated schema output.
• Verify pinned versions table is current. The RC versions table was carried over from the previous doc — confirm the versions still match what's in the package.json files after the v5 migration work.

Notes

No functional testing needed — this is a markdown-only change with no code impact. CI passing confirms no build regressions.

Requested by: @pyramation
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