fix(summoner): preserve user-set agent name after summon completes

[teexiii]

Problem

When creating a predefined agent with a custom name, the name was
overwritten in two places once summoning completed:

1. display_name in the DB was replaced by the LLM-extracted name from
   IDENTITY.md (e.g. "Tiểu La"), ignoring the user's input.
2. IDENTITY.md itself still contained the LLM-generated name, so the
   agent introduced itself with that name regardless of what was stored
   in display_name.

Root cause

finishSummon unconditionally applied extractIdentityName(identityContent)
to the display_name update map with no check for an existing value.
Even if the user had set a name at creation time, both the DB field and
the IDENTITY.md persona file ended up with whatever name the LLM chose.

Fix

• Before writing the LLM-derived name, load the current agent record.
  Only update display_name if it is empty (user did not provide one).
• If the user did set a name that differs from the LLM-generated one,
  rewrite the Name: line in IDENTITY.md via replaceIdentityName() so
  the agent's persona file matches the user's intended name.
• Regeneration (ResummonAgent) is left unchanged — when explicitly
  rebuilding a persona, updating the name is expected.

Result

An agent created as "La Alilala" will have both display_name = "La Alilala"
and Name: La Alilala in IDENTITY.md after summoning completes, and will
introduce itself with the correct name in chat.

[viettranx]

UX concern đã được fix trong commit ecf6463:

Trước: Summon/regenerate/resummon ghi đè display_name vô điều kiện từ LLM → user mất tên custom đã đặt.

Sau: Nếu user đã đặt tên custom → giữ nguyên display_name + sync IDENTITY.md Name: cho khớp. Agent tự giới thiệu đúng tên user chọn, UI cũng hiện đúng. Nếu chưa có tên (tạo qua API) → dùng tên LLM như cũ.