Reduce artifact bundles to a minimal kernel with typed evidence refs

Parent: #2429

## Problem

Artifact bundles currently encode manifest integrity, metadata, events, commands, observations, logs, mounted inputs and outputs, diffs, changed files, patches, review summaries, tests, runtime references, replay indexes, replay status, snapshots, browser evidence, and workflow-specific projections.

Many are useful evidence types, but treating every projection as part of the mandatory artifact protocol couples the runtime to reviewer, replay, benchmark, browser, agent, and product concerns. It also creates repeated result-normalization code across CLI and WordPress surfaces.

## Desired shape

Define a small mandatory artifact kernel:

- immutable bundle identity and file manifest;
- runtime, input, policy, and tool provenance;
- canonical workspace delta;
- normalized command/step result records;
- redaction/integrity verification status;
- typed references to optional evidence.

Replay, browser, tests, benchmark, fuzz, review, and product-specific evidence should be optional typed artifacts referenced by the kernel. The runtime should not duplicate those projections into several top-level envelopes.

## Acceptance criteria

- One minimal artifact-kernel schema is sufficient for every successful, failed, interrupted, and no-op run.
- Optional evidence is discoverable through typed references without mandatory placeholder fields.
- Workspace changes have one canonical representation; patches and reviewer summaries are derived views.
- Bundle verification validates the kernel and referenced evidence without knowing every extension schema.
- CLI, PHP, and Homeboy consume the same artifact result projection.
- Existing bundles needed for persisted compatibility remain readable through a bounded migration reader; new writes use only the canonical format.
- Redundant artifact result DTOs and normalization paths are removed after consumer migration.

## AI assistance

- **Model:** OpenAI gpt-5.6-sol
- **Tool:** OpenCode
- **Used for:** Inspected the documented artifact lifecycle and implementation surface and drafted the minimal kernel plus extension-evidence contract under Chris Huber's direction.

Non vengono nominati file o test. Inizia leggendo il ciclo di vita documentato dell’artefatto e la superficie di implementazione a cui fa riferimento l’issue, quindi traccia i consumer CLI, PHP e Homeboy. Il lavoro è completato quando sono soddisfatti i criteri di accettazione: un unico kernel canonico, evidenza opzionale tipizzata, letture di compatibilità limitate e nessun DTO dei risultati o percorso di normalizzazione ridondante.

Scritto dal modello di indicizzazione a partire dal testo della issue.