feat(export): per-community subfolders + path-aware filenames for to_obsidian

[fitliferepo]

Why

When to_obsidian is run on a real codebase, the current output has three issues that get in the way of using the vault:

Issue	Before	After
Filename collisions	__init__()_1.md, __init__()_2.md (no way to tell which file)	app-routers-pipeline-py--init.md, app-models-py--init.md (path-traceable)
Flat vault	All N node notes in one folder; hard to browse	One subfolder per community, _OVERVIEW.md inside
Cross-community links	[[_COMMUNITY_X]] (dangle once you rename overviews)	[[folder/_OVERVIEW|name]] (always resolves)

Tested on a ~1,700-node codebase: 0 dangling wikilinks, 0 lost files, 64 navigable subfolders instead of 1,700+ files at root.

What

graphify/export.py:

• to_obsidian: pre-computes a unique folder name per community id (handles label collisions), routes each node note into its community folder, replaces _COMMUNITY_*.md with per-folder _OVERVIEW.md, emits a top-level _INDEX.md with communities sorted by size.
• to_canvas: uses the same path-aware naming and emits canvas file refs as <folder>/<filename>.md so the canvas resolves against the new layout.
• Filename derivation: <last-3-source-path-segments-slug>--<label-slug>. Falls back to plain label slug if source_file is missing, then to node if both are empty.

Tests

4 new tests + 1 updated:

• test_to_obsidian_writes_per_community_subfolders — subfolder + overview + index emit
• test_to_obsidian_filenames_disambiguate_by_source_path — no _1 suffixes for same-label nodes
• test_to_obsidian_unique_folders_when_community_labels_collide — three communities labelled "main" → main/, main-2/, main-3/
• test_to_obsidian_has_no_dangling_wikilinks — vault-wide invariant
• test_to_canvas_file_paths_relative_to_vault — updated to assert canvas refs resolve to real files on disk

All 19 tests in test_export.py pass. Full suite (test_export.py test_cli_export.py test_build.py test_cluster.py test_analyze.py): 84 passed. Pre-existing failures in test_multilang.py (SQL) and test_ollama.py are unrelated and reproduce on stock v7.

Compatibility

• to_obsidian and to_canvas keep the same signatures and return types.
• Downstream tooling that opens the directory as an Obsidian vault is unaffected — Obsidian resolves [[bare-name]] links across subfolders automatically.
• If users have callers that read _COMMUNITY_*.md at the vault root, they'll need to switch to reading <folder>/_OVERVIEW.md.

[safishamsi]

Thanks for this — per-community subfolders is a great UX improvement and the test coverage is solid. Two issues to fix before we merge:

1. Duplicated helper code (must fix)
_slug, _node_stem, and the folder-precompute block are duplicated verbatim between to_obsidian and to_canvas. If they ever drift, canvas refs will silently point to the wrong files. Please extract them to module-level helpers shared by both functions.

2. Cross-folder wikilink collisions (must fix)
used_in_folder only deduplicates stems within a folder, so two nodes in different communities can share the same <path>--<label> stem. Wikilinks are written as bare [[neighbor_label]], so Obsidian's shortest-path resolver picks one arbitrarily on collision. The existing test_to_obsidian_has_no_dangling_wikilinks test passes either way because it only checks that some file with that stem exists, not that it's unique. Please either (a) enforce global stem uniqueness across all folders, or (b) write wikilinks as fully-qualified [[community_folder/stem]] paths.

Minor

• _INDEX.md uses community_labels.get(cid, ...) raw in [[folder/_OVERVIEW|name]] — if a label contains | or ] the wikilink breaks. Worth sanitizing.

Once these are fixed and the PR is rebased onto current main (v0.7.19), we'll merge. Thanks!