[Bug] rc.6: every tool call fails with "unknown tool """ — streamed deltas drop the tool name (write/bash all broken)

[21Ssss]

Severity: blocker — the agent cannot perform any tool call (write file, run command, etc.) in web UI and headless mode.

Version: 0.1.0-rc.6 (npm latest, macOS arm64, Node v26). Reproduced in both --profile web and --profile headless.

In every session since install, all tool calls fail with Error: unknown tool "". The model clearly receives the task and emits a correct first delta — but the harness loses the tool name/id after the first streamed delta.

Log evidence (from session.jsonl.zstd, one write call):
// 1st delta — correct: {"type":"assistant/chunk","chunk":{"type":"tool-call-delta","index":1,"id":"call_591b3387062d4706b2dc6079","name":"write","argumentsDelta":""}} // 2nd+ deltas — name/id wiped: {"type":"assistant/chunk","chunk":{"type":"tool-call-delta","index":1,"id":"","name":null,"argumentsDelta":"{"file_path": ...}"}} // assembled call → empty name/id: {"type":"tool/call","data":{"callId":"","name":"","arguments":"..."}} // executor: Error: unknown tool "" (ToolNotFoundError / UNKNOWN_TOOL)

Live repro (web UI, 2026-08-17): asked it to echo bug-demo-2026. It understood the task, invoked bash, got "unknown tool """, retried 5+ times with different invocation formats (system even injected repeat-tool-reminder ×5), all failed; 34s / 8 steps / 66.5K input tokens wasted on a doomed retry loop.
Headless repro: cd $(mktemp -d) && npx @deepseek-ai/dsh --profile headless "write hello.txt with hello" → exits 0, prints nothing, no file created (silent failure).

Suggestion: when merging streamed tool-call deltas, keep the first delta's name/id (later deltas only carry arguments); add an e2e test that splits one tool call across multiple deltas.