feat(gfql): add Plottable schema accessor

[lmeyerov]

Closes #1632.
Refs #1058.

Summary

• Add experimental read-only g.schema accessor returning the locally bound GraphSchema or None.
• Keep the predicate surface minimal: callers can use g.schema is not None.
• Add anchored bound/unbound/read-only/chained regression tests and docs/changelog coverage.
• Harden agents/skills/review/SKILL.md so future GitHub PR/issue Markdown bodies are posted through body files instead of inline shell strings.

Design Notes

• The accessor returns the exact object supplied through bind(schema=...); it does not copy, infer, mutate, or pretty-print schemas.
• Scope is intentionally local: g.schema does not fetch/hydrate remote dataset schemas, persist schemas to Nexus, or serialize schemas into gfql_remote() requests in this release.
• Per-type contracts such as Cat, Dog, and Car stay in GraphSchema.node_types: NodeType.name is the stable public type identity and NodeType.labels are GFQL label predicates such as label__Cat.
• Future remote/backend work should transport a versioned graph-schema envelope derived from GraphSchema.to_arrow() beside gfql_query / gfql_operations, with exact Arrow schema IPC plus a JSON summary for Nexus/viz/REST consumers.
• Local design receipt: plans/plottable-schema-accessor-1632/design/schema-transport-design.md.
• No top-level graphistry.schema() accessor was added because graphistry.schema is already the public schema module/export surface and this is plotter-state introspection.
• Compiler-plan surface touched: no.
• DGX RAPIDS 25.02/26.02: not run; this does not change GPU/cuDF/dataframe runtime behavior.

LOC Buckets

• Production: +24 / -1, net +23 LOC
• Tests: +24 / -0, net +24 LOC
• Docs/process/changelog: +59 / -3, net +56 LOC

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/test_public_schema.py -> 24 passed
• python3 -m pytest -q docs/test_doc_examples.py -k schema -> 1 passed, 29 deselected
• ./bin/ruff.sh graphistry/PlotterBase.py graphistry/tests/compute/gfql/test_public_schema.py -> pass
• ./bin/typecheck.sh graphistry/PlotterBase.py graphistry/Plottable.py graphistry/schema.py -> pass
• git diff --check -> pass
• Review skill converged: 2 original clean waves plus post-design/rebase and has_schema() removal follow-up waves, no findings; artifacts at plans/plottable-schema-accessor-1632/final-report.md
• Full PR CI green on rebased run 26421509425: changed-line coverage, RTD/docs gates, TCK GFQL, GFQL core, docs, full-AI, core-python, Spark, Neo4j, pandas/polars/graphviz, and broad matrix jobs passed.

Sample Usage

schema = graphistry.GraphSchema(...)
g = graphistry.bind(schema=schema)
assert g.schema is schema
assert g.schema is not None
assert graphistry.bind().schema is None

Merge Readiness

Ready for manual review. Branch is rebased over latest origin/master; CI is green; review is required before merge.