feat: add read-only JSON memory CLI for SSH/cron automation

[manager-identity]

Summary

• Adds a sandbox-friendly read-only memory surface for LoopLens/Hermes (SSH + cron) that opens ai-memory/brain.db directly in SQLite read-only mode (PRAGMA query_only = ON) without shelling through Doppler.
• New commands: pt memory search --query ... --json, pt memory recent --json, pt memory stats --json, pt memory export --format ndjson, pt config show --effective --json, pt doctor --json, pt --version.
• Existing human pt memory search "query" behavior is preserved; JSON/filter mode uses the new direct read-only path.

Design notes

• Stable schema: every JSON response carries a schema_version string (pt.memory.v1, pt.config.v1, pt.doctor.v1).
• Distinct exit codes for automation: 2 validation, 3 backend unavailable, 4 query failure, 5 auth.
• Asymmetric error routing: single-object commands (search/recent/stats) emit error JSON to stdout (consumer branches on ok); memory export emits errors to stderr so its NDJSON stdout stream stays exclusively a data channel. _emit_json_error is annotated NoReturn to make the control flow legible to type checkers.
• Migration warning gating is unchanged from main: _warn_unapplied_migrations writes to stderr only, banner is TTY-gated, and PT_SUPPRESS_MIGRATION_WARNING=1 is the documented opt-out for cron/SSH.

Test plan

• PT_SKIP_DOPPLER=1 ./pt doctor --json returns a valid pt.doctor.v1 payload with all checks ok on a healthy box.
• ./pt memory recent --since 7d --limit 5 --json returns a pt.memory.v1 envelope with stable pagination fields.
• ./pt memory export --project project-tracker --format ndjson prints one JSON object per line on stdout, nothing on stderr; exit code 0.
• PT_MEMORY_DB_PATH=/tmp/missing.db ./pt memory export prints nothing on stdout, a structured backend_unavailable JSON object on stderr, exit code 3.
• ./pt memory search "MCP" --top 3 (no --json, no filters) still hits the existing brain.py path and behaves identically to main.
• ./pt info list --json (existing command) still emits the same output as main — confirms the migration-warning gate change did not regress unrelated --json consumers.
• pytest tests/test_memory_cli.py — 8/8 green.

Related

• Closes #6059