Allow relative paths on more verbs, cascade real deletes, refine context defaults

[oskaresparza]

Summary

• create_folder/data_copy/data_move/set_wiki/delete_wiki/get_wiki/set_tags/delete_tags/get_tags/delete_table/create_view/delete_view now all accept either an absolute path or one relative to the current context, matching use()'s own resolution rules — previously most of them only took absolute paths, which broke on Dremio's Arctic-backed source (a 400, not 404, on a by-path lookup into a not-yet-existing nested item) once a custodian tried a bare create_folder call.
• delete_folder's cascade=True now actually deletes tables/views underneath too, via the same SQL DROP TABLE/DROP VIEW delete_table/delete_view already use, instead of the generic catalog-REST delete (which Dremio doesn't seem to trust with real data).
• set_wiki ignores its tags parameter when path is a table/view, since those have native tags instead of the wiki "Meta Data" convention meant for folders.
• CatalogSession now starts with context at the catalog root ("catalog") instead of None, so a custodian's first relative call already has something to resolve against.
• use(None)/use("") reset context back to that same root rather than clearing it to no context at all (that's what set_context(None) is still for — the lower-level primitive, not normally called directly).
• use() also prints context set to <path> itself now, since it never queues anything for the usual commit-report auto-print to show.
• Updated %catalog help, README, the design doc, and both example notebooks to match throughout.

Test plan

• pytest — 371 passed
• ruff check / ruff format --check — clean on touched files

🤖 Generated with Claude Code