feat: add ataegina list --json (fleet view)

[noahhyden]

What & why

Fifth and final planned agent-native addition (docs/design/agent-native.md). A supervising agent that spawns workers across worktrees needs to see the whole fleet in one structured call. list --json emits the registry as a JSON array — the shared slot shape per worktree (primary first) plus stale and live:

[{"index":0,"repo_root":"…","frontend":{…},"backend":{…},"log_dir":"…","db":null,"stale":false,"live":false},
 {"index":1,"repo_root":"…-wtA","frontend":{"port":5174,…},…,"stale":false,"live":true}]

• stale — the worktree directory is gone (run prune).
• live — coarse best-effort: any of its derived ports is listening (always false under a none port backend).
• Human list output is unchanged.

_ate_list_entry_json computes a slot for an arbitrary (index, path) from the repo's config (port bases, DB naming) rather than the current tree, reusing emit_db_json for each entry's db object.

Discipline

• Spec: docs/design/agent-native.md.
• Tests-first: tests/list_json.bats — 10 hermetic tests: array shape, per-worktree ports, stale (a removed worktree dir), live (fake ss on a derived port), per-worktree db naming (unsuffixed primary + wtN), single-primary case, human-output regression, arg validation, completion. Completion assertions across ports/status/list made robust to future group changes.
• Live verification: drove the binary across a primary + two worktrees and parsed the array with python -m json.tool.
• Mutation red-teaming: stale-always-false, live-never-true, and db-suffix-dropped — each caught by a distinct test.
• Docs: README, man page, CHANGELOG, bash+zsh completion, in-script --help. Checksum regenerated. No new dependencies.

Series

Completes #27 (ports --json), #28 (up --wait), #29 (status --json), #30 (up --json).

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