feat(cli): stream incremental reasoning/text deltas in run --format json

[mustafa3rsan]

Issue for this PR

N/A — no existing issue.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

opencode run --format json --thinking never streams reasoning. During a long thinking phase the JSON stream is silent, then the whole reasoning block appears at once when it ends.

The loop in run.ts only handles message.part.updated, and gates reasoning/text on part.time?.end, which is only set once the part finishes (SessionProcessor.finishReasoning). The incremental tokens are published as message.part.delta (session.updatePartDelta), but run never listens for them, so they're dropped.

This adds one branch that emits message.part.delta as part_delta events when --format json. A partID -> part.type map (filled from message.part.updated) tags each delta so consumers know whether it's reasoning or text:

{"type":"part_delta","partID":"prt_...","field":"text","delta":"...","partType":"reasoning"}

emit() is a no-op unless --format json, so the formatted/TTY output is untouched (it still waits for the whole part on purpose). It's a new event type, so existing JSON consumers ignore it; the final reasoning/text events are still emitted at time.end.

How did you verify your code works?

Ran the patched build from source against a real model:

bun dev run --thinking --variant high --format json --model anthropic/claude-opus-4-8 "..."

The JSON stream now includes part_delta events (it didn't before): 4 part_delta / 2 reasoning / 2 text / 1 step_start / 1 step_finish. Reasoning tokens arrive incrementally instead of only at block end, e.g.:

{"type":"part_delta","partID":"prt_f8f0...","field":"text","delta":"I","partType":"reasoning"}
{"type":"part_delta","partID":"prt_f8f0...","field":"text","delta":"'m calculating that 17 times 23...","partType":"reasoning"}

bun run typecheck (tsgo --noEmit) passes and oxlint reports 0 errors on the file.

Screenshots / recordings

N/A — not a UI change.

Checklist

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

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[github-actions]

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