Why: per-environment bring-up needs a persistent file for the SVC_*
knobs (model keys, addresses, tavily pool) instead of relying on shell
history or the single global .svc.env; CLI flags must still win for
one-off overrides like a different port.
Evidence: live matrix - .env.dev parsed (comments, paired quotes,
export prefix) with its address and model values applied and seeded;
--port 8831 overrode the file's port while keeping its 0.0.0.0 host;
missing file fails fast; .svc.env and --env layer correctly (overlap
keys resolve to --env, disjoint keys merge).
Impact: --env FILE / --env=FILE anywhere on the command line; dotenv
parsed safely line-by-line (valid identifiers only, never sourced);
precedence CLI > --env file > .svc.env > OS env > defaults; help and
make svc ARGS examples updated.
Refs: feature/svc-envfile task card.
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feature/svc-envfile：--env 环境配置文件
scripts/svc.sh dev --env ./.env.dev [--port 9000 ...]。dotenv 安全解析（注释/引号/export）；优先级 CLI > --env 文件 > .svc.env > 系统环境变量 > 默认值；与模型播种贯通。
实测：文件地址+模型生效并播种、--port 覆盖文件端口保留文件 host、缺文件快错、.svc.env 与 --env 正确分层合并。