[Bug] Session is named "Untitled session" when pasting a large amount of text from the clipboard (it becomes a .txt attachment).

提交前确认 · Pre-submission checklist

• 我已搜索过现有 issue，确认这不是重复 / I searched existing issues and confirmed this isn't a duplicate.
• 我已阅读 CONTRIBUTING.md / I've read CONTRIBUTING.md.

问题类别 · Category

对话 / Agent 交互 · Agent chat

涉及的 Agent 框架 · Agent framework

ZCode Agent（自研）

严重程度 · Severity

影响体验 · Major (功能可用但体验受损 / works but degraded)

复现频率 · Reproducibility

必现 · Always

问题描述 · Description

When the first (and only) user message in a session is a large pasted text,
ZCode converts that text into a .txt file attachment (a file message part)
instead of an inline text part. The session therefore ends up with an empty
or file-name-based title, and in the Command Palette / session list it is
displayed as "Untitled session".

Root cause (traced in the v3.6.5 bundle):

1. Paste handler (renderer composer). When pasted plain text is >= 5 KB
   (threshold cJe = 5*1024), it is converted to a temp attachment via
   createTempTextAttachment({ text, filename }), where filename is
   generated as pasted-text-YYYYMMDD-HHMMSS.txt, mimeType text/plain,
   sourceKind clipboard-text. So the user's input becomes a file part,
   not a text part.

2. Title derivation ignores file parts. textFromZCodeMessageParts filters
   message parts by type === "text" only — file parts are skipped.
   resolveZCodeVisibleSessionTitle therefore finds nothing and falls back
   to "New session" / "Untitled".

3. createTask title fallback is a filename. deriveSessionTitle(content, attachments) returns the first 50 chars of inline text, but when there
   is none it returns attachments[0].filename ("pasted-text-*.txt"). This
   is then typically overwritten by the LLM title generator; if that
   generation is disabled (e.g. cron automations set titleGenerationEnabled: false) or fails/returns empty, the title degrades and aqe() in the
   Command Palette renders the hardcoded literal "Untitled session".

Net effect: any session whose first message is only a large paste ends up
nameless in the session list, making history hard to navigate.

复现步骤 · Steps to reproduce

1. Open ZCode and start a new session in any workspace.
2. Copy a piece of text of >= 5 KB (e.g. ~6000 chars of code/docs) to the
   clipboard.
3. In the chat composer, paste (Cmd+V) the text WITHOUT typing any other
   text alongside it.
4. Confirm the pasted content is rendered as an attached file
   "pasted-text-*.txt" rather than as inline text in the input box.
5. Send the message.
6. Open the Command Palette / session list and look at the title of the
   newly created session — it appears as "Untitled session" (or briefly as
   "pasted-text-*.txt" until the LLM title generation completes; if the
   generation is absent or fails, it ends up as "Untitled session").

期望表现 · Expected behavior

The session title should be derived meaningfully from the pasted content —
for example, from the first lines of the .txt attachment, or at minimum
from its filename — and should never appear as "Untitled session".

Ideally, the text of a clipboard-text attachment (sourceKind:
"clipboard-text", mimeType: "text/plain") should be treated in title
derivation the same way as inline text message parts.

实际表现 · Actual behavior

The session shows up as "Untitled session" in the Command Palette. In the
sidebar / task list it may also appear as "New task" (i18n key
taskList.untitled) or as "pasted-text-*.txt", but degrades to
"Untitled session" in the palette when the LLM title generator does not run
or returns empty.

ZCode 版本 · ZCode version

v3.6.5

设备 / 系统 / 浏览器 · Device / OS / Browser

macOS (darwin 25.6.0, arm64) / ZCode Desktop App

截图 / 录屏 / 日志 · Screenshots / Recordings / Logs

No response

Start with the v3.6.5 bundle's paste handler and the title paths named in the report: textFromZCodeMessageParts, resolveZCodeVisibleSessionTitle, deriveSessionTitle, and aqe(). Reproduce the large clipboard paste case, then verify that a clipboard-text attachment produces a meaningful session title and never "Untitled session" when title generation is disabled or empty.