feat: add ataegina ports --json (agent-native slot output)

[noahhyden]

What & why

First of five agent-native CLI additions (see docs/design/agent-native.md). Ataegina's audience is coding agents running fleets of worktrees; they parse output programmatically, so the human table in ports is friction. This adds ataegina ports --json.

ataegina ports --json prints a single line of JSON — the shared slot object the rest of the surface (status, up --json, list --json) will reuse:

{"index":1,"repo_root":"/abs/wt","frontend":{"port":5174,"url":"http://localhost:5174"},"backend":{"port":8001,"url":"http://localhost:8001"},"log_dir":"/tmp/ate-wt1","db":{"name":"myapp_wt1","url":"postgres://localhost:5432/myapp_wt1"}}

• db is null when no database is configured; db.url is null when DB_NAME is set without a DB_URL_TEMPLATE.
• Plain ports (no flag) keeps its human output unchanged.
• Two reusable helpers land here: json_str (bash-3.2-safe JSON string escaper, backslash-first ordering) and emit_slot_json.

Discipline

• Spec: docs/design/agent-native.md (covers all 5 features).
• Tests-first: tests/ports_json.bats — 11 hermetic tests (shape, per-worktree ports, db variants, quote/backslash escaping stays valid JSON, human-output regression, completion advertises the flag).
• Live verification: exercised the real binary end-to-end, piped through python -m json.tool for primary + worktree ataegina: add move command + report what down stops #1 with a DB configured.
• Mutation red-teaming: 3 deliberate mutations (db-always-null, skip backslash-escape, index-as-string) — each caught by a distinct test.
• Docs: README command table, man page, CHANGELOG, bash+zsh completion.
• No new dependencies; single bash file preserved.

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