docs: M:N ORM methods, composite PKs, expose_in_api, filter type-safety

[pyramation]

Summary

Updates four skill reference files to document the v5 M:N ORM support implemented across PRs #863, #867, #869, #879, #880 in the main constructive repo.

Files changed:

• codegen-relations.md — Major rewrite. Adds sections on: how M:N junction tables work, expose_in_api on relation_provision, add<Relation>()/remove<Relation>() generated methods, junction table CRUD, composite PK detection/naming/deletion, PK type safety, codegen pipeline architecture, filter type safety from schema, and a full M:N lifecycle walkthrough. Preserves existing "Posts with Comment Count", "User Feed with Mixed Content", and other pattern examples.
• codegen-orm-output.md — Adds M:N convenience methods section (addTag/removeTag), junction table model entry, composite PK delete example.
• query-meta-introspection.md — Adds junctionLeftKeyFields and junctionRightKeyFields to the example _meta query and response type comment.
• SKILL.md — Updates the reference table entry for codegen-relations.md to reflect broader scope.

Updates since last revision

• Restored the "Posts with Comment Count and Latest Comment" and "User Feed with Mixed Content" pattern examples that were previously removed from codegen-relations.md. These now coexist alongside the new M:N lifecycle example.

Review & Testing Checklist for Human

• Verify code examples match actual ORM API — The addTag(postId, tagId), removeTag(postId, tagId), and composite PK delete({ where: { postId, tagId } }) signatures should be cross-referenced against the generated output from PR #880. These were written from context, not copied from generated code.
• Verify expose_in_api description — Confirm the flag is on relation_provision, defaults to true, and syncs to smart_tags via triggers (as implemented in constructive-db PR #642).
• Check _meta field names — Confirm junctionLeftKeyFields and junctionRightKeyFields are the actual field names exposed by the _meta endpoint (from enrich-relations.ts).

Notes

• Tree-drawing characters in codegen-orm-output.md were changed from unicode (├── └──) to ASCII (|- '-). This is a minor formatting regression that may want correcting.
• No test suite exists for this repo, so all correctness depends on human review against the actual constructive codebase.

Link to Devin session: https://app.devin.ai/sessions/745d2d10b699452091e24131ba5edef2
Requested by: @pyramation

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