Fetch graph schema live via APOC, cached for the session (1.4.0)

[adamjohnwright]

The curator reported Claude saying the APOC schema was "limited." Root cause: reactome_cypher_schema was calling the sparse built-in db.schema.*, which omits counts, cardinalities, and indexes. When Claude tried apoc.meta.schema() via reactome_cypher_query, the ~500 KB single-row result hit the per-row token cap and returned a truncation stub.

Fix: pull the rich schema inside fetchGraphSchema, format it ourselves, never surface it through the capped query tool.

What changed

• src/graph/schema.ts: new module. fetchGraphSchema runs apoc.meta.{schema,stats,nodeTypeProperties,relTypeProperties}, dbms.components, db.indexes, db.constraints in parallel and assembles a GraphSchema object. Cached in-memory for the session; concurrent first-callers share one fetch via a pending-promise. 60 s timeout per query (up from the 30 s cypher default) because apoc.meta.schema() samples 3M nodes.
• src/graph/format-schema.ts: markdown digest with labels sorted by node count, relationship types sorted by cardinality, per-label + per-rel property types with mandatory flags, indexes, constraints. ~80 KB on Reactome (was ~40 KB with db.schema.*).
• src/index.ts: prefetch on startup so the first tool call is warm.
• src/tools/cypher.ts + src/resources/static.ts: route to the new module. No functional change beyond what's documented.
• Split out of src/clients/neo4j.ts so tests can vi.mock runRead across the module boundary.
• 7 new tests: format coverage + cache/dedup/fallback behaviors.

Verified live against the running Reactome Release96 container:

• Cold fetch: 21 s on the full graph (this is why we prefetch).
• Second call: 0 ms, same object reference.
• Markdown digest: 80,123 chars.
• Mocked-fallback test confirms graceful degradation when apoc.meta.relTypeProperties / db.indexes / db.constraints are unavailable.

No vendored schema artifact. No coordination with reactome_neo4j_env. One source of truth: the live database.