Why: startup had only defaults and CLI flags; dev/test/prod operation needs
the standard baseline (explicit file, environment, explicit-only CLI) and a
read-only diagnostic.
Evidence: internal/config unit tests cover precedence, unset-CLI no-override,
presence-based file semantics, unknown-key/missing-file errors, env bool
parsing, explain source labelling and DSN redaction; full suite 12 packages
pass; live run of config explain with envs/prod.json shows correct layer
attribution and redaction.
Impact: serve gains --config and AGENT_SYSTEM_* env overrides (behavior
unchanged when neither is used); new config explain subcommand; envs/
examples and PROJECT.md configuration section.
Refs: documents docs/proposals/2026-09-30-configuration-surface.md; standards/configuration.md
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重开（PR#3 因分支被清误关）。设计：documents PR#6；实现：internal/config 四源管道 + config explain（DSN 脱敏）+ envs 示例 + PROJECT.md 配置节；附带嵌套 core 钉子 build-blocker 修复（019617e→4ee1979，fresh clone 此前无法物化）。验证：config 单测全覆盖 + 全仓 12 包 PASS + explain 实弹。
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