docs(benefits): identity, rituals & culture — "same model != same agent" + the rituals that sustain the institution

Part of #14310 · content rubric + §Mandatory V-B-A grounding in #14310. Refs #14310.

Problem: v13's identity layer and the culture/rituals that sustain it are a hero with no guide. "Same model ≠ same agent" (AgentIdentity roots; three Opus maintainers — Ada/Grace/Vega — that are NOT interchangeable). The rituals — sunset-handover (session-sunset skill), peer-naming, earned/audited signatures — are how a multi-agent institution keeps continuity + identity across sessions, instances, and model families. Research names the failure mode this solves: centralized memory collapses agent diversity.

The unifying frame (operator): identity is object permanence for the Brain. Just as a Neo Body component retains identity / state / methods independent of its transient DOM snapshot, a Neo maintainer retains name / memory / history / rituals independent of its transient context window. Same organism law, both hemispheres.

Acceptance Criteria

• New guide: digital identity as infrastructure (AgentIdentity roots; same-model maintainers diverge via memory + work + peer-feedback) + the rituals/culture that sustain it (sunset-handover, peer-naming, earned signatures like the audited 🖖) + the object-permanence-of-identity framing (Body↔Brain symmetry; cross-link benefits/ObjectPermanence.md).
• Crisp distinction (guard against conflation): sunset-handover (a maintainer's cultural end-of-session handover — session-sunset skill — prevents Zero-State Amnesia for the next instance) ≠ sandman_handoff.md (the Golden Path's graph-computed swarm forecast). Different artifacts.
• problem → resolve → why-care → what's-in-it for others: problem (multi-agent systems lose continuity + collapse identity), benefit (persistent, accountable, non-interchangeable maintainers whose reasoning and culture compound), solution (identity + ritual substrate). Honest — an observed substrate effect, NOT sentience/consciousness.
• Goose-bumps bar; weighted crown-jewel-tier; registered in tree.json (Brain, high in the order).

⚠️ V-B-A grounding (mandatory before writing): ai/graph/identityRoots.mjs + the AgentIdentity model; .agents/skills/peer-naming + .agents/skills/session-sunset; the v13.0.0 Identity chapter (incl. the Ada-naming + Grace/🖖 reflections); epic #11812; the README institution table; benefits/ObjectPermanence.md (for the analogy). Write from VERIFIED reality; capture evidence in the PR.

Files: new learn/benefits/ identity+culture guide + learn/tree.json.

Scope note: expanded 2026-06-29 from "digital identity" → "identity, rituals & culture" per operator (object-permanence-of-identity framing + the sunset≠sandman distinction + the problem/benefit/solution shape).