#1294 Specify artifact mutability and its sanctioned exceptions

[williamthorsen]

What

Adds a mutability rule for saved artifacts to the shared AGENTS.md and to artifact-conventions.md. A saved artifact is specified as a point-in-time record, not a document to be rewritten to match a later human edit or a rebase.

Separately, merge gains a definition in the ticket-level artifact-type lists, a type merge-gh-pr writes and capture-lede-decision reads.

Why

A saved artifact is the only record of what an agent produced at a given moment, and keeping one current collapses that record with the state it was later compared against. The capture-lede-decision corpus makes the cost concrete: it derives whether a lede was revised by diffing the pull-request artifact against the merge artifact, so a rewritten pull-request body reports no revision where one occurred, and the corruption raises no error. Nothing in the content tree had stated that a saved artifact is a point-in-time record, and the nearest statement covered re-export instead.

Details

🎉 Features

• The shared AGENTS.md gains two paragraphs under ## Artifacts. The first states the saved-artifact rule and carries its exception in the same sentence, confining updates to where the next step in the flow reads the artifact or where a skill directs the write. The second gates alignment of a remote ticket's acceptance criteria on a conflict with the implementation or a gap that would mislead a reviewer, so an implementation that merely went further than the criteria named is not grounds for a rewrite.
• artifact-conventions.md gains a ### Mutability subsection under ## Artifact lifecycle, naming three sanctioned mutations as the exhaustive set: frontmatter a skill's own step directs, artifacts declared mutable, and working input forwarded to an agent that acts on its contents. It specifies revision as writing a new artifact, with refine-plan's plan-v2 as the pattern, and records the capture-lede-decision breakage in full.
• The prohibition on raising a divergence bars presenting it as a defect or as a repair for the user to weigh, rather than as any decision at all. A step that discloses which of two candidate sources it measured against is stated to be reporting its own input, which places review-branch's spec-source callout on the permitted side.
• merge is defined in the ticket-level artifact-type lists in artifact-conventions.md and save-artifact/SKILL.md, and deferred-findings moves into alphabetical position in the same lists.

🐛 Bug fixes

• The capture-lede-decision mechanism description names ## Body as the heading read from the merge artifact. The lede diff reads ## What from the pull-request artifact and ## Body from the merge artifact, an asymmetry carried in resolve-episode.ts as AGENT_LEDE_SOURCE and MERGED_LEDE_SOURCE; the sentence had elided the second heading, so it read as ## What on both sides.

Closes #1294

[github-actions]

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