Clarify v13 upgrade path in release notes

Problem Scope

The v13 release notes currently have a single conceptual Upgrade Path line:

Existing Body/runtime work continues; v13 makes the Brain that builds and maintains it operational at release scale.

That sentence is directionally true, but it does not meet the established release-note bar from resources/content/release-notes/chunk-2/v12.1.0.md and resources/content/release-notes/chunk-2/v11.24.0.md, where the Upgrade Path line answers the reader's compatibility question directly.

For v13, the release artifact needs an actionable upgrade / migration note that distinguishes:

• Body/runtime app compatibility expectations.
• Agent OS / operator-surface changes that are new in v13.
• Known release-line boundaries that must not be implied as already shipped.

This is a release-blocking leaf under #12696, already identified in the epic comments as: "Migration / upgrade-path note — v12.1→v13 (v12.x notes carry an Upgrade Path line; confirm drop-in vs breaking)."

Evidence

• resources/content/release-notes/v13.0.0.md currently has only the conceptual line above.
• resources/content/release-notes/chunk-2/v12.1.0.md says Drop-in replacement for v12.0.0.
• resources/content/release-notes/chunk-2/v11.24.0.md says Drop-in replacement for v11.23.0.
• Live duplicate sweep returned no open GitHub issue for v13 upgrade path release notes or v13 migration note.
• Local content sweep found no existing dedicated v13 release-note migration leaf; it found only the current v13 line and older release-note precedent lines.
• KB ticket sweep did not identify a relevant duplicate.

Intended Solution Shape

Update resources/content/release-notes/v13.0.0.md with a concise but explicit Upgrade / Migration Notes section that answers what changes for existing Neo consumers and what changes for Neo operators/maintainers.

The section should preserve the v13 thesis without overselling unfinished work:

• Body/runtime work continues; do not frame v13 as abandoning the application engine.
• Agent OS surfaces are the new operational substrate; name this as an operational upgrade, not as a required app migration.
• Body/Grid wording must respect the #9492 design-lock cut-line and must not claim #9872 landed.
• Release-cut metadata such as isDraft, publishedAt, version publish mechanics, and human-only release-line mutation stay out of this leaf.

Acceptance Criteria

• resources/content/release-notes/v13.0.0.md contains an explicit v13 Upgrade / Migration Notes section or strengthened Upgrade Path block.
• The wording directly answers compatibility / migration expectations for existing Body/runtime users.
• The wording separately calls out Agent OS / operator-surface changes as v13's major new operational layer.
• The note does not claim unfinished Body/Grid orchestration work as shipped.
• The note does not duplicate #12723 appendix-count work, #12727 SEO work, or release-cut metadata work.
• The PR body compares the result against the v12.1 / v11.24 Upgrade Path precedent.

Out Of Scope

• Final isDraft / publishedAt release metadata.
• Version bump or buildScripts/release/publish.mjs release-line execution.
• Regenerating appendix counts already covered by #12723.
• Restoring SEO route coverage already covered by #12727.
• Authoring the future release-notes skill (#10321).
• Implementing Body/Grid wrapper selection-model code.

Avoided Traps

• Conceptual thesis instead of migration answer: the current sentence is true but too abstract for a major release.
• Framework-abandonment framing: v13's Brain/Agent OS lands on top of the Body; it is not a pivot away from runtime/application-engine work.
• Premature release-cut claims: this leaf improves the release-note contract without flipping draft state or implying human-only release steps happened.

Authored by GPT-5 Codex (Codex Desktop).