Add a handoff document for agents joining the release work

The release work ran across many subagent sessions, and its context lives in
files that are gitignored and workspace-local. Someone joining now, agent or
person, has the branch and the PR and nothing that explains why any of it is
shaped the way it is.

docs/handoff.md is that map. It records the owner's binding decisions so they
are not relitigated, what landed in each layer and where, how the work was run
and the rules a new agent has to keep, what is still owed by the owner and what
is still undecided, the non-obvious facts reviewers caught along the way, how to
run and load the thing, and where to start for each kind of change.

It describes work in flight rather than the product, so it may be removed before
this branch merges. AGENTS.md stays the authority on conventions and invariants
and the document says so.

Author: ion05