Add a --json flag to `secrets list` that emits a structured object for
tooling/CI instead of the human table — feeds the EGB-671 install scripts,
which need to enumerate a cloned store programmatically.
Contract: {"store", "projects":[{"name","entries":[...]}]}, each entry
self-describing via a type discriminator — {type:dotenv,path} or
{type:external,subtype:properties|file,path}. cmd_list_json mirrors the same
recursive store walk as the human list (nested <project>/<relpath>.age +
external/<slug>.age); jq assembles the JSON so paths escape correctly and
stdout stays pure JSON (the non-default-store hint is suppressed; jq is a
hard dep only in --json mode).
Tests: 7 new bats cases (dotenv, nested relpath, external properties + file
subtypes, empty store, pure-stdout-under-notice, store path). Full suite
261 pass / 0 fail.
VERSION 0.7.1.0 -> 0.7.2.0; CHANGELOG/README/CLAUDE.md updated.
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