Add support for condensing full.jsonl -> transcript.jsonl (opencode and claude first)

[computermode]

Pulling out changes that have been tested from #754 to keep the diffs smaller.

With these changes, Opencode and Claude full.jsonl files can be transformed to a uniform transcript.jsonl format that only includes what the UI needs. The changes have also been validated with the UI: the generated transcript.jsonl format is parse-able for both of these agent types.

You may notice that the Claude changes are in a compact.go file: that's because that file will include what all the agents have in common. Since Claude is the default and has a decent amount of crossover already with its full.jsonl with other agents, that's why its code is within that shared file. The Opencode agent parser uses some of the helper methods from it.

Below are screenshots from the UI where I supplied the code with real full.jsonl files and generated transcript.jsonl files that I manually pushed up to the checkpoints v1 branch on a test repo (the metadata.json had to be updated to route to transcript.jsonl as well of course).

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Note

Medium Risk
Introduces new transcript-compaction logic that changes how tool results and streaming assistant fragments are merged/omitted, which could affect downstream UI parsing across multiple agent formats.

Overview
Adds a new transcript/compact package that converts agent transcripts into a normalized transcript.jsonl format with stable field ordering and per-line metadata (v, agent, cli_version).

For JSONL-based agents (Claude Code/Cursor), the converter now filters non-semantic event types, strips assistant thinking/redacted blocks and extra tool fields, merges streaming assistant fragments by message ID, and inlines tool_result outputs into preceding tool_use blocks while dropping tool-result-only user lines.

Adds OpenCode support by detecting the single-JSON session export format and emitting equivalent compact user/assistant lines (including tool parts and timestamps), plus comprehensive fixtures and unit tests covering truncation, mixed formats, and edge cases.

^{Written by Cursor Bugbot for commit f672fc9. Configure here.}

[Soph]

and #793