* fix(rag): skip hidden and junk directories when indexing (#5559)
index_personal_documents walked the whole tree with no pruning, so
pointing RAG at a real-world folder silently swept in .obsidian/ plugin
JS, .git/ internals, node_modules/, and __pycache__/ — multiplying
indexing time and polluting retrieval with junk chunks.
Prune hidden directories and well-known junk directories from the walk,
and skip hidden files. The explicitly passed root is exempt, so a user
who deliberately indexes a hidden directory still gets its contents.
* fix(rag): prune hidden/junk dirs in the keyword index too, via a shared helper
The #5559 fix pruned only VectorRAG.index_personal_documents (the vector index).
The parallel keyword index built by PersonalDocsManager.refresh_index ->
load_personal_index walked the same tree unpruned, so .obsidian/, .git/,
node_modules/ etc. still swept into keyword retrieval and the file listing —
the 'end-to-end' guarantee was only half true.
Single-source the pruning policy in src/index_walk (prune_index_dirs +
is_indexable_file) and use it from both walkers so they cannot drift again.
The junk-dir match is now case-insensitive, so a Node_Modules on a
case-insensitive filesystem is pruned too.
Tests: keyword-path regressions covering hidden/junk dirs, hidden files, junk
at depth (not just top level), case-insensitive junk, and the explicit-hidden-
root exemption. The existing vector tests still pass against the shared helper.
