vedit-core: add stable merge command contract for source/optional-target and merge result metadata

[explicit09]

Summary

Awidat has requested a stable merge command contract for caller tooling (CLI/TUI/desktop) with predictable output for audit UI and reasoning logs.

Requested in vedit-core

• A standard merge command signature:
  • source_ref
  • optional target_ref (default current checked-out branch/commit)
• Deterministic merge result payload including:
  • source,
  • target,
  • new commit,
  • merge parents,
  • changed clip IDs,
  • optional commit/header metadata suitable for downstream logs.

Why

Awidat command surfaces currently need a single, portable object from core instead of assembling partially in local wrappers.

Acceptance criteria

• API returns structured errors for not-found, unmerged preconditions, and conflicts.
• Output is directly serializable for desktop/agent command layers.