Introduces a pure, FUSE-independent item tree (buildItemTree) shared by the mount and export commands. The new export subcommand downloads Docspell search results into a local directory, mirroring the mount's friendly folder/year/month hierarchy without the by-id directory or metadata JSON file, for archival or migration.
- itemtree.go: pure item tree builder shared by mount and export
- mount.go: buildMountTree now converts the pure tree into FUSE nodes, with a shared fileNode per attachment across friendly and by-id paths
- export.go: export command with concurrent downloads, --dry-run, --skip-existing, --concurrency, --query, --limit, --auth-header
- main.go: wires up the export subcommand and its flags
- export_test.go: tests for plan collection, writes, skip-existing, failures, modtime
- README.md: documents the export command
