backdraft 0.10.0

A week on saying what happened in a form an agent can act on — and one on taking something back.

• backdraft forget <slug> withdraws a source that ingest should not have taken: out of read, search and ls, with nothing removed, so a token already written into somebody's draft or artifact still shows its receipt and bind reports it unresolved naming the withdrawal rather than passing it silently. Ingesting the source again is the undo, as the same document under the same slug.
• backdraft session show reads the ledger back: a total and one line per document, so "have I read enough to write this yet?" is answerable before the draft exists rather than by binding it and counting not_shown. It also says out loud, at exit 0, what the shared default session costs.
• An unreadable source now says what to do about it instead of relaying an errno — a directory wants the files inside it, a missing path wants its spelling checked, an unreadable file wants its permissions fixed — and a source with no bytes in it is a failure rather than a document with nothing to cite. Same rule for the URL half, including a server that sends an empty body.
• The markdown projection bind --bound writes now calls a fetched source by its page, the way ingest, ls, the gate's list and the artifact already did. That is the form that travels into a pull request or an email, where the artifact cannot follow.
• An unknown slug is told about the sources a withdrawal hid, rather than being told the registry is empty.

1340 tests.