feat: add ataegina status [--json] (read-only liveness)

[noahhyden]

What & why

Third of five agent-native additions (docs/design/agent-native.md). Agents need a side-effect-free way to ask "is my stack up, and where?" before acting, to stay idempotent (avoid double-up, know the pid/port). status is that query.

Per surface it reports:

• running — a live server ataegina launched holds the port
• foreign — something ataegina did not launch holds it (a collision)
• unknown — the port is held but ownership can't be verified (often our own reparented daemon)
• stopped — nothing on the port

Starts/stops nothing, opens no DB connection, and exits 0 regardless of state (state is data, not failure). --json folds state+pid into the shared slot shape:

{"index":1,"repo_root":"…","frontend":{"port":5174,"url":"…","state":"running","pid":1234},"backend":{"port":8001,"url":"…","state":"stopped","pid":null},"log_dir":"…","db":{"name":"myapp_wt1","url":"…"}}

Reuses _ate_port_ownership + ate_port_pids; factors the shared db sub-object into emit_db_json (now used by both ports and status). Also backfills the in-script --help (usage()) for ports --json and up --wait — the prior two PRs updated the man page/README but not usage().

Discipline

• Spec: docs/design/agent-native.md.
• Tests-first: tests/status.bats — 10 tests. stopped/foreign/unknown hermetically via a fake ss (no real socket); a read-only assertion (status must not create the log dir, unlike up); db object; per-worktree index/ports; arg validation; completion. Plus an integration test proving a real python server we launch reads as running.
• Live verification: drove the binary for stopped (human + python -m json.tool) and confirmed the real-server running path end-to-end.
• Mutation red-teaming: swap foreign→running, pid-always-null, and not-listening→running — each caught by a distinct test.
• Coverage: the only non-hermetic branch is running (needs a real owned process), covered by the integration job; ~20 lines of headroom over the 99% gate (was 99.84%).
• Docs: README table, man page, CHANGELOG, bash+zsh completion, and usage(). Checksum regenerated.

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