feat(agents): agents.runtime_command, and acp becomes a runtime you can name (#1634)

[jhgaylor]

Opens the door the previous PR built behind. acp joins the runtime enum, a
new runtime_command column carries the command it launches, and model
becomes optional for that one runtime.

• The column. agents.runtime_command is text: a value may be a whole
  shell line ("cd /srv/app && exec bin/agent acp"), and a length rule belongs
  in the changeset if one is ever wanted. agents.model loses its NOT NULL in
  the same migration, because whether a model is required is a rule about the
  pair of columns and belongs where both are in hand.
• Both refusals are 422s naming the field. A missing runtime_command on
  acp, or one carried by any other runtime, which resolves its own
  executable from a pinned table. A runtime_command that is stored and never
  run reads as configuration and is not.
• A model is still checked when it is given. Optional on acp and inert
  there, but it still has to parse and name a known provider, because a value
  that is stored and ignored is worse than one that is refused.
• model is nullable on the wire, in the response and in both request
  bodies, so an agent converted to acp can clear the model it no longer uses
  with {"model": null}. CastAndValidate runs before the changeset, so a
  non-nullable string there would reject the null with a 400 and leave the
  stale value on the row forever. AgentRequest drops model from required
  and the changeset enforces it instead, so a missing model is a 422 rather
  than a 400.
• Model suggestions return nothing for acp: advice to fill in a field that
  does nothing is worse than no advice.
• The export carries the field, so an account export stays a restorable copy.

Nothing runs a turn on this runtime any better than before this PR — the
dispatch that spawns it landed below, and the skills mount, the refusal path,
the console and the docs are above.

sdk-no-release: contract.json and the generated TypeScript types move
here, and the version bump is the last PR in the stack.

---

Stack for #1634 (merge top down; each PR is based on the one below it in the list):

	PR	What it is
1	#1832	Fountain.CommandRuntime and the host dispatch for acp
2	#1833	agents.runtime_command, and acp becomes a runtime you can name
3	#1834	skills mount on a runtime the library does not know
4	#1835	a turn on the acp runtime, end to end
5	#1836	refuse a turn whose agent no longer carries a command
6	#1837	the acp runtime in the agent form
7	#1838	a real ACP program through a whole turn
8	#1839	docs, CHANGELOG and SDK 1.25.0

Replaces #1677, which was the same feature as one 1,760-line branch.

🤖 Generated with Claude Code

https://claude.ai/code/session_01A2iMoNvSvrkQQfnP4RzVi2

[jhgaylor]

The first run of A change to the SDK says whether it releases failed because the run started about two minutes before the sdk-no-release label landed on this PR. The label is what the gate's own error message asks for when a PR changes the published surface without releasing, and the version bump is #1839. Re-running.