`basert serve`: support loading an external chat template (`--chat-template`)

[chaoshou-coder]

Problem. baseRT's built-in chat templates reject mid-list role:"system"
messages. The official Qwen3.5/3.6 template does:

Qwen/Qwen3.6-35B-A3B, commit 7da1103:
{%- if message.role == "system" %}
    {%- if not loop.first %}
        {{- raise_exception('System message must be at the beginning.') }}
    {%- endif %}
{%- endif %}

baseRT catches the exception and silently falls back to a generic ChatML
template: the request succeeds but loses model-specific formatting, tool-call
syntax, and reasoning delimiters. Agentic coding tools (Claude Code, and
others) inject role:"system" reminders mid-array at runtime, so this affects
every agentic workload on a Qwen3.5/3.6 model.

Request. A --chat-template <path> flag (or equivalent, as llama.cpp
provides) to load an external chat template at serve time, so deployments can
use community templates without re-converting models.

Reference. A community fix already exists:
froggeric/Qwen-Fixed-Chat-Templates
v21.3 (MIT) — replaces the exception with per-position ChatML rendering of each
system block, and adds agentic features (tool truncation, consecutive-failure
detection, inline thinking control). A reference copy and analysis are in the
cc-bridge PR (cc-bridge/misc/chat-template-fix.md and
chat_template-patched.jinja).

Impact. Fixing this at the engine level removes the system-merging
workaround from downstream bridges (cc-bridge's --no-system-merge exists for
this) and lets users deploy agentic-optimized templates without converting.

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