std/github: interact with PRs and stacks through github api

[nishu-builder]

Second of three, stacked on #300 (the design). This adds std/github. The next PR removes /pr.

After the review on #300, this replaces the first version of this PR, which built a github tool into llm-step and made publish_source refuse a .caos entry. Both are gone. Net, the PR is the new tool plus a few lines of agent guidance.

std/github is a std tool like any other. The agent runs it with run_tool(path="caos-std/github"), and tool_help describes it. It's a Go worker on std/go, whose image carries the CA bundle. One run is one call to api.github.com.

• Arguments: method, path, an optional JSON body, and at. They're checked before anything is sent: the method; an API path (a full api.github.com URL is accepted); no body on a GET; a body that is JSON.
• Result: the status line, then the body, cut at 100 kB on a character boundary. A response of any status is a result. The run fails only when no response came back, and for a write it then says the request may still have arrived.
• at is required. The tool has no @in, so its ArgTree is its arguments, and a call repeating all of them returns the stored result. (salt is the interpreter's.)
• Token: /secret/github-token, granted by a second reader: line naming std/github. Without it, only public reads work. The host is fixed and redirects aren't followed, so the token goes nowhere else.

Also in this PR:

• The agent guide opens PRs and links a stack with caos-std/github, and no longer suggests /pr.
• publish_source's help points at the tool.
• std/README.md lists the tool.

Testing

• Worker, locally. I built worker.go with three changes for the test: the API base, /cas/args and the secret path read from the environment. It ran against a local stand-in server with a fake caos on PATH:
  • a POST was sent once, with its body verbatim (including a -1.5 that canonical JSON couldn't hold) and the Accept, API-version, Bearer and JSON content-type headers;
  • a GET with no token sent no Authorization, and a 401 got the no-token hint;
  • a 302 was reported and not followed (the redirect target saw no request);
  • a 120 kB body was cut at 99,999 bytes and stayed valid UTF-8;
  • a PUT with no body sent Content-Length: 0;
  • a bad method, a missing at, a GET with a body, a relative or spaced path, and a non-JSON body were each refused before sending;
  • a dead server failed the run, with the may-have-arrived advice for a write.
• Build and lint: the file as committed passes go build, go vet and gofmt. llm-step's 46 tests pass.
• Not run:
  • The caos suite: its nix build needs flake inputs from GitHub repos this session can't reach. There is no stack test for the tool: covering a real call would need the worker to accept another API host, which would let the token leave api.github.com.
  • A live call to api.github.com, so the stacks endpoint is still unexercised.

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https://claude.ai/code/session_01CZGkTta5NYxhXcLPZwH3q1