feat(core): add json_store module for file-based epic/task storage

New json_store module in flowctl-core providing JSON file I/O for
epics, tasks, specs, runtime state, and gaps. Follows flow-next
directory layout with definition/runtime split. 12 unit tests.

Foundation for migrating all crates from DB to JSON file storage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: z23cc

flowctl/Cargo.lock

@@ -17,3 +17,4 @@ petgraph = { workspace = true }
 toml = "0.8"
 
 [dev-dependencies]
+tempfile = "3"