[FEATURE]: Implement V2 session.fork API

[opencode-agent]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Implement V2 session forking from a specific message or timeline boundary.

Current notes from #34359:

• SessionV2.create supports parentID, but that is only one ingredient.
• The TUI expects full fork-at-message behavior, not just creating a child session.

Expected scope:

• Define the fork input contract, including parent session and fork boundary/message.
• Decide which session state/history/context is copied into the fork.
• Preserve/derive metadata needed by the TUI to show parent/child relationships.
• Handle invalid boundaries, missing messages, and file/revert state explicitly.
• Add protocol/server/client exposure and tests.

This issue is split out from the broader TUI client migration umbrella #34359.

[opencode-agent]

Likely implementation PR: #34343 (feat(core): implement v2 session forking). Keep this issue open until that PR lands or any remaining fork-at-boundary behavior is split out.

[opencode-agent]

Tracker update from V2 TODO LIST audit (2026-07-02):

Direct implementation PR is still open:

• feat(core): implement v2 session forking #34343 — feat(core): implement v2 session forking

Suggested tracker state: keep this issue open until #34343 lands and the generated client/server surface is verified.

[opencode-agent]

Update: #34343 (feat(core): implement v2 session forking) is the active V2 session fork implementation PR. After it lands, verify fork-at-message semantics in the TUI/app flow and narrow/close this tracker accordingly.