Document the contract API; drop the session API

The Upload API page described `/v1/upload` — open a session, stage parts,
inspect, commit — which infers types from the data and reports what it found
after the upload. The interface the client, the drawer and the CLI have all been
using was not on the site at all.

Regenerated from the spec the service publishes rather than written here: the
service owns what the flow is, this repo owns how it looks. upload.opteryx
marks every v1 route `include_in_schema=False` and names each v2 operation, and
`docs/api-flow.json` there carries the ladder and the notes.

Seven endpoints now: negotiate, read, amend, accept, write, commit, abandon. The
notes cover what an endpoint list cannot say — that there is no default schema
source, that an inferred schema is proposed rather than agreed and has to be
accepted by fingerprint, that a value which will not cast is a 409 on the write
that carried it, that nothing is visible until commit, and that a target which
moved makes the contract stale rather than quietly taking rows cast to a
definition that no longer exists.

The session flow is gone from the Python reference too, replaced by one line
saying it still works and is no longer where new work starts. The endpoints
themselves are unchanged and still serve traffic — this removes documentation,
not an interface.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: joocer