Migrate panto CLI onto the public.zig surface

Move the CLI off the internal libpanto module namespaces onto the curated
public API: data-type aliases (Config family, Message/MessageRole/
effectiveSystemBlocks, Event, Pricing/PricingRegistry, the session seam,
FileSystemJSONLStore, ContentBlockType), process lifecycle (panto.init/
deinit), and ResultParts.fromText/fromTextOwned/deinit in place of the
freestanding textResult/ownedTextResult/freeResultParts.

The CLI remains on two flagged internal namespaces, panto.agent and
panto.conversation: it is a deep embedder that drives the loop below the
curated Agent/Conversation facades (system-prompt seeding through the
agent, compaction_system_prompt, the open_stream_fn test seam, standalone
Conversation values, compactAndPersist). These stay re-exported from
public.zig as a documented deep-embedder escape hatch; trimming the
transitional block removed every other internal re-export.

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