Session.fork does not remap CompactionPart.tail_start_id, breaking filterCompacted and inflating forked context #24700

Description

Session.fork copies all messages and remaps messageID and parentID via an idMap, but does not remap tail_start_id inside CompactionPart parts.

Root cause
src/session/session.ts:596–602 — the part spread includes the raw (old) tail_start_id:

for (const part of msg.parts) {
  yield* updatePart({
    ...part,                          // spread includes un-remapped tail_start_id
    id: PartID.ascending(),
    messageID: cloned.id,
    sessionID: session.id,
  })
}

filterCompacted (src/session/message-v2.ts:1072) relies on tail_start_id:
retain = part.tail_start_id // ← old message ID, never remapped

Forked session has new message IDs → msg.info.id === retain never matches → break never hit → all pre‑summary messages unfiltered.

Impact
Forked session context inflates massively (~300k → ~800k tokens observed per API-Usage response).

Fix
Remap tail_start_id in the fork loop via idMap, analogous to parentID (lines 588–593).

Plugins

no

OpenCode version

fresh clone

Steps to reproduce

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Screenshot and/or share link

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Operating System

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Terminal

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