Prompt format entity

A CanonicalPromptFormat is CanonicalEntity<"promptformat", PromptFormatBody>. The body is name plus two
optional halves, instruct and context, each a flat record of ST's own field names to scalar values, and optional
notes. SillyTavern binds an instruct template and a context template by name, so one piece holds both. The type
lives in src/entities/promptformat/schema.ts.

Nothing applies a prompt format yet. Bunzai talks to chat-completion connections, which take role-tagged messages
and need no hand-written turn markers; a prompt format is for text-completion connections, which Bunzai does not
have. The piece is stored whole so the files are not lost. The Library card says so in one computed line, which is
never written into the piece. There is no editor: opening one on the Workbench shows that line and the fields, read-only, and a wireframe for a real viewer
is at design/wireframes/features/prompt-format/index.html.

An extension may stage reviewed edits to the stored name, instruct/context scalar fields, or author notes through
content.draft.promptformat.templates.update. These fields remain inert canonical data; this operation does not apply
the templates to prompt assembly. Draft review and apply own confirmation and persistence.

Fields

Field	Type	Meaning
name	string	The format's name; also the name inside each half on export.
instruct	record	ST instruct template: input_sequence, output_sequence, system_sequence, stop_sequence, wrap, names_behavior, and every other key ST wrote.
context	record	ST context template: story_string, example_separator, chat_start, trim_sentences, and every other key ST wrote.
notes	string	The author's own notes.

Import and export

The sillytavern-promptformat codec reads either file or ST's master wrapper. Pairing a same-named instruct and
context into one piece happens in the Library import (pairPromptFormats). Export writes one file for one half
and the master wrapper for two. See formats/sillytavern.md.