feat(opencode): add markdown output to export command

[molloyzak13]

Issue for this PR

Closes #9387

Type of change

• New feature

What does this PR do?

opencode export only emitted JSON. This adds -f/--format json|markdown and -o/--output <file>, so a session can be saved as a readable Markdown transcript instead of only JSON. The default stays json, so existing usage and scripts are unchanged. The markdown path walks the v1 part union (text, reasoning, tool) and writes a session-header table plus each turn. It also removes a MessageV2 import in the same file that was already unused.

How did you verify your code works?

Built the CLI (packages/opencode/script/build.ts --single) and ran the binary against a real session: export <id> -f markdown (stdout), -f markdown -o <file> (writes the file), and plain export <id> to confirm the JSON output is byte-for-byte unchanged. tsc --noEmit is clean.

Screenshots / recordings

Not a UI change (CLI output).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[molloyzak13]

Small heads-up on the diff: I also dropped the MessageV2 import on line 3. It was already unused before this PR — once I edited the file the typecheck flagged it as dead, so I cleaned it up rather than leave it sitting there. It is a one-liner and unrelated to the markdown logic, so if you would rather this PR stay purely additive I am happy to pull it into a separate change.