docs(skills): add graphile-postgis skill for @spatialRelation + RelationSpatial

[pyramation]

Summary

Adds a new .agents/skills/graphile-postgis/SKILL.md so future Devin sessions (particularly agentic-DB sessions, which were the original motivator for the PostGIS work) reach for the right primitive when a task involves spatial queries.

The skill covers:

• When to use it — triggers like "find clinics in a county", "client is pulling GeoJSON to filter in JS", "about to write a per-pair helper SQL function".
• Tag grammar for @spatialRelation + the 8-operator reference table (including the st_dwithin parametric case).
• Blueprint shape for the new RelationSpatial node (one-direction-per-tag, idempotent, metadata-only).
• ORM where: usage — some / every / none semantics, parametric distance:, and composition with and/or/not.
• Self-relations (single vs composite PK exclusion, distance: 0 corner case).
• GIST indexing and the @spatialRelationSkipIndexCheck escape hatch.
• Debugging checklist seeded with the concrete failures from shipping this feature (column reference "name" is ambiguous, bare vs metaschema-modules:-prefixed pgpm.plan refs, smart-tag clobber, some: {} semantics, SRID vs meters).
• Scope guardrails listing common wrong turns (custom resolver taking a polygon, per-pair helper functions, mixing codecs, using in orderBy).
• Pointers to plugin source, README, blueprint node, and the E2E/unit test suites.

Also adds a one-line cross-link to the new skill from constructive-monorepo-setup/SKILL.md under the "Graphile Plugin Development" section so it's discoverable from the monorepo entry-point skill.

No code changes. Plugin README already contains the full reference (from #994) — this skill is deliberately a shorter, task-oriented summary that points at the README for deep details.

Review & Testing Checklist for Human

• Content accuracy — skim the operator table and tag grammar against graphile/graphile-postgis/README.md and graphql/node-type-registry/src/relation/relation-spatial.ts to confirm the skill doesn't drift from the source of truth.
• Relative link correctness — the skill references ../../../graphile/graphile-postgis/... and ../../../graphql/node-type-registry/... (three levels up from .agents/skills/graphile-postgis/). Click through once to verify GitHub renders them.
• Tone / scope for an agent consumer — this is written for Devin sessions, not humans. Flag anything that reads more like product marketing than actionable instructions.

Notes

• The skill deliberately repeats a subset of the README rather than linking out exclusively — agent sessions often need self-contained guidance without a second fetch.
• The "Debugging checklist" symptoms are lifted from real failures encountered while shipping constructive-io/constructive-db#844 (parameter ambiguity) and the pgpm cross-module reference issue in the same PR, so they'll appear verbatim in future sessions.
• Phase C (agentic-DB prompt update) is a follow-up that now has a concrete URL to cite.

Link to Devin session: https://app.devin.ai/sessions/c5eeee65a3c546c4ac6753bb05fa03e0
Requested by: @pyramation

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