feat(mcp): run a bootstrap flight when no flight exists to select

[jlrickert]

Second of four. Targets #82; retargets to main once that merges.

A session that can reach no flights at all had nothing to offer: the KEG tools
were locked and the recovery text asked the user to select a flight that did not
exist. Sessions now run on a synthetic bootstrap flight whose cover is empty —
every KEG operation stays denied — but which grants manage_flights and
manage_kegs, so the agent can create the first flight and the first KEG and
then be selected into real authority.

The provider decides whether to synthesize it, because only it knows how to
count its transport's flights; the gate reads that decision off the manifest.
Bootstrap sessions expose an allowlist rather than the full tool set, so the
agent does not spend the session discovering the same refusal one tool at a
time. Recovery instructions ride in the manifest because the local and hosted
surfaces nudge toward different places.

The payload never calls this an "active flight": a reader who believes a flight
was selected will not go set one up, which is the entire point of the mode.

keg_create joins the agent surface for this, gated on manage_kegs and
re-checked inside the tool so an embedded surface without the session gate
cannot reach creation through a flight that never granted it. A new KEG is not
readable until a flight's cover names it.

Also here

• fix(keg): node 0 is the placeholder landing node and carries no type by
  design, so requiring one made every schema-bearing KEG permanently invalid —
  and the standing error told agents to "fix" node 0 by destroying it. Scoped to
  that one rule; a node 0 that does declare a type is still validated.
• feat(cli): contextWindow and args per launch agent. Harnesses express a
  context cap differently, so the launcher translates per harness and reports
  rather than drops it where there is no equivalent.
• docs(mcp): orientation, node 0, and attachment paths move into the rules
  block, which is what survives a context reset. The plural assets/ and
  images/ spelling is asserted, since a singular path yields links that upload
  fine and then resolve to nothing.

Testing

Every commit builds, passes go test ./..., and is gofmt-clean on its own
(verified with git rebase --exec).