feat: add FalkorDB export backend (#1175)

[galshubeli]

Closes #1175.

Adds FalkorDB as a sibling option to the existing Neo4j sink, selected via
graphify export falkordb (with --push for a direct push).

What

• New push_to_falkordb() in graphify/export.py — mirrors push_to_neo4j().
• export falkordb subcommand wired in graphify/__main__.py, parallel to neo4j:
  writes cypher.txt with no --push, pushes directly with --push.
• falkordb optional extra in pyproject.toml (and added to the all extra).
• Tests in tests/test_cli_export.py (cypher generation, CI-safe) and a new
  tests/test_falkordb_integration.py (real FalkorDB, auto-skips when none reachable).
• README extras table + command reference and CHANGELOG updated.

How

FalkorDB is OpenCypher-compatible, so the MERGE/SET upsert queries are identical
to the Neo4j path — the function is a near line-for-line mirror of push_to_neo4j.
Differences are documented in the function's docstring:

• connects with FalkorDB(host, port, username, password) (URI parsed for host/port,
  default port 6379) instead of a bolt driver, and selects a named graph via
  db.select_graph(graph_name) (default "graphify");
• queries run via graph.query(cypher, params) — no session object;
• auth is optional (FalkorDB runs without credentials by default), so the push path
  connects anonymously unless a password is supplied;
• no APOC — the Neo4j path doesn't use it either, so nothing to port.

A couple of notes on scope, to keep the diff tight and reviewable:

• graphify has no sinks/ package or --graph-backend flag today — the Neo4j sink is
  a function in export.py dispatched by the export <format> subcommand. I mirrored
  that exact pattern rather than introducing new structure.
• I deliberately did not add a top-level --falkordb skill flag (the
  --neo4j/--neo4j-push flags live in the generated skill files under skillgen's
  drift guard). Happy to add that as a follow-up if you'd like parity there.

Test plan

• pytest tests/test_cli_export.py tests/test_export.py — passes (no external service).
• pytest tests/test_falkordb_integration.py against falkordb/falkordb:latest — passes
  (push + node/edge count assertions + idempotency); auto-skips with no FalkorDB.
• Manual: indexed test fixture pushed with graphify export falkordb --push redis://localhost:6379 produces 4 nodes / 4 edges, equivalent to the Neo4j path, and
  is idempotent on re-run.

— Gal