docs(runtimes): the acp runtime, ADR 0049 and SDK 1.26.0 (#1634)

[jhgaylor]

The last PR of the stack. Everything the seven below it built is now
documented and released.

• docs/catalog/runtimes/index.md gains the runtime, a row in the
  comparison table, a skills-layout row, and Running a program as an agent:
  the manifest, the five things that follow from the command being a shell
  line, and how the turn is priced.
• Two traps have their own paragraphs, because both are silent and both
  cost a turn. Write exec before the program, or the login shell stays as the
  parent and an interrupt can leave the program running on a persistent
  sandbox. And nothing may print on stdout before the program starts, because a
  profile's banner lands in front of the first protocol message.
• What "the same isolation" means is stated where it is weakest. The free
  string is safe because it runs where an environment's setup script already
  runs. On sandbox_provider: runner with the default backend that is a
  directory and the daemon's own user, so the page says so and links to
  trusted mode.
• docs/concepts/agent.md, docs/concepts/permissions.md (both tables, since
  whether the command asks and how long a grant lasts are its decisions, not
  Fountain's), docs/sdk.md, docs/reference/glossary.md, docs/build/index.md
  and the three "all four runtimes speak ACP" lines.
• SDK 1.25.0. Agent["model"] is string | null, and the two request
  bodies take string | null optionally. A client that assumed a string needs
  a null check; nothing else on the wire changed shape.

The docs suite, docs-style.py (its two findings are pre-existing pages I did
not touch), vale and destink all pass.

---

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

[lex00]

Two things before this one goes in:

• CHANGELOG.md line 50 has a stray >>>>>>> theirs marker that this commit introduced (it is in none of the source branches).
• main moved to SDK 1.25.0 with #1809 after this stack was cut, so this release collides; it needs 1.26.0, and #1849 then needs 1.27.0 or the other way round depending on which lands first.

Approving the rest of the stack.

[jhgaylor]

Rebased onto the current main. #1809 landed while this stack's CI was running and took 1.25.0 for the @managoat/fountain-sdk rename, so this bump is now 1.26.0 and its changelog entry sits above main's. All four edit sites moved together: package.json, package-lock.json (two spots), src/http.ts's USER_AGENT and CHANGELOG.md.

Worth noting the collision was invisible to the tooling: GitHub reported MERGEABLE the whole time, because two releases claiming the same version number is a semantic conflict and not a textual one.