feat: add native code edit event to session transcript

[NathanFlurry]

Summary

Add a first-class/native code edit event in the session transcript so clients can render file edits directly without reconstructing them from tool output text.

Problem

Today, code edits are mostly inferred from tool output and post-hoc diffs. This is brittle and provider-dependent.

Proposal

Introduce a dedicated session event/item shape for code edits (or an equivalent normalized event) that includes structured edit metadata.

Suggested minimum fields:

• file path
• operation (create/update/delete/rename)
• before/after hashes (if available)
• patch/diff payload (or structured hunks)
• tool/source attribution
• timestamps / sequencing compatible with existing item.started -> item.delta -> item.completed model

Why

• Makes UI rendering of edits deterministic
• Avoids parsing free-form tool text
• Improves parity across providers and event consumers
• Simplifies downstream integrations that need edit timelines

Acceptance criteria

• Session events expose a structured code-edit event for actual file mutations
• Event shape is documented in docs/session-transcript-schema.mdx
• Conversion docs are updated (docs/conversion.mdx)
• HTTP/SSE tests cover the new event end-to-end

Related: #102