Why: beyond database binding, services needed a way to switch the JSON config
file they run with and have the new config take effect promptly; hand-editing
exec= risks restarting a service into an invalid config.
Evidence: in the task worktree with a real agent binary -- a config with an
unknown key is rejected by the binary's strict schema check before any state
change; a valid file binds via --config in exec= + conffile= (survives
record_state rewrites), restarts the running service, and the config's dsn
verifiably drives the process (wrong-password variant failed PG auth in the
log; correct variant connected and created 21 tables); editing the file and
re-running 'config <same file>' re-validates and restarts (pid changes);
'--clear' removes both the flag and conffile; db and config coexist in exec=;
view mode shows masked JSON content plus 'config explain' effective values
with masked dsn; list DB column also resolves dsns that live only inside the
config file. Full log in the task card.
Impact: new service-level action 'svc.sh <name> config [file | --clear]
[--no-restart]' for agent services; config-file content snapshots (masked,
--reveal to show) in info; a config-file line in status; conffile persisted
in svc.env. No agent binary changes.
Refs: task card .task/feature-svc-db-bind.md (same doc as PR #182);
docs/svc.md sections 2/4.5/4.6/6/10.1
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