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
Why: main pinned 019617e, a pre-rewrite chain SHA reachable only via the
rewrite-backup archive ref; fresh clones cannot materialize it (the earlier
advance fixed the worktree state but staged the pre-image pin).
Evidence: git ls-tree shows 019617e on origin/main while core advertises
4ee1979 as main; local builds passed only because the clones carry the
pre-image objects from the chain-bundle fetches.
Impact: build-blocker fix folded into this feature branch; pins core at its
current main.
Refs: documents/records/2026-09-30-history-migration.md
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