FIX-13: local opt-in, metadata-only structured logging for CLI/app server

[ChelseaKR]

Add on-device, opt-in structured JSON logging for the two local surfaces
(the habitable CLI and the loopback app server), mirroring the optional
relay's metadata-only discipline (src/habitable/relay.py). It exists so an
operator debugging their own device can trace what happened without ever
emitting anything that must stay on-device.

• New src/habitable/obslog.py: a shared _JsonFormatter (one compact JSON
  object per line: ts/level/msg + an explicit event_fields map),
  configure_logging() installing it idempotently on a dedicated habitable
  logger (propagate=False), enabled_from_env() (only HABITABLE_LOG=json
  opts in), and log_event() — the privacy gate: it is a no-op until logging
  is configured and refuses any non-scalar field, so a dict/list/bytes/Path
  payload can never ride onto a log line.
• CLI: top-level --log-format json flag; logging enabled by the flag or
  HABITABLE_LOG=json; a metadata-only command-boundary event (name, ok,
  duration_ms) — never arguments, paths, ids, or secrets. Off by default,
  emitted to stderr so stdout stays clean.
• App server: honors HABITABLE_LOG=json; per-request access lines use a
  redacted _route_label (e.g. /api/issues/{issue}/timeline) so no issue
  id, static path, or query value is logged, and never a body.
• capture.py / sync.py: emit metadata-only events (capture result booleans,
  redundant-authority counts, sync ciphertext sizes and merge counts).
• tests/test_obslog.py: formatter shape, env gating, the scalar-only guard,
  CLI end-to-end JSON to stderr, off-by-default silence, app-server redacted
  routes, and a no-plaintext guard paralleling tests/test_relay.py — a real
  init/capture/export flow with sentinel passphrase, filename, media bytes,
  and case text asserts none of them (nor the device fingerprint) appear in
  the log stream.
• ROADMAP.md: mark the observability item shipped (FIX-13).

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

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Backfill PR for a completed roadmap item (roadmap/fix-13-local-opt-in-structured-logging-f, 1 commit(s) over main). Part of the portfolio roadmap batch.