feat: introduce codex-pr-body skill

[bolinfest]

Motivation

Codex needs a repeatable workflow for updating PR metadata after a pull request already exists. This is more specific than generic GitHub handling: the assistant needs to preserve author-provided body content, explain why the PR exists before listing implementation details, and describe only the net change under review, including when Sapling stacks put a PR on top of another PR instead of main.

Changes

• Adds .codex/skills/codex-pr-body/SKILL.md.
• Documents how to infer the target PR from the current branch or commit, including Sapling-specific PR metadata and sl sl output.
• Defines the expected PR body update behavior: inspect the existing body, preserve key content such as images, avoid local absolute paths, use Markdown formatting, include relevant issue/PR references, and call out developer docs follow-up only when applicable.
• Captures stacked-PR handling so generated PR text describes the change between the PR's base and head, rather than unrelated ancestor changes.

Verification

Not run; this is a Codex skill documentation addition.

[bolinfest]

I should mention that sl sl is equivalent to sl, but this:

prefix_rule(
  pattern = ["sl", "sl"],
)

is safer than this:

prefix_rule(
  pattern = ["sl"],
)

since the latter allows all Sapling commands, not just the smartlog.