feat(cli): default amy stdout to human-readable text; --json opts in

amy's default stdout is now a YAML-ish render of the underlying result
map; the previous single-line JSON contract moves behind a global
`--json` flag. Errors mirror the same rule (`error: <code>: <detail>`
on stderr by default, JSON `{"error":...,"detail":...}` under --json).
Exit codes (0/1/2/124) and the --json shape itself are unchanged —
only the default presentation flips.

- Replaces Json.writeLine / Json.error with mode-aware
  Output.emit / Output.error. The same Jackson mapper is reused for
  on-disk JSON via Output.mapper.
- Adds `--json` to the global flag set in Main.kt; honoured even when
  argument parsing fails so error JSON keeps shape under --json.
- Updates the test harness wrappers (amy_a / amy_b / amy_d in
  cli/tests/{headless,dm,cache}/) and the few direct $AMY_BIN call
  sites whose stdout is consumed via $() / 2>&1 — they now pass
  --json so the existing jq pipelines keep working.
- Rewrites the README "Output contract" and DEVELOPMENT design
  principles to describe the new default, and clarifies that only
  --json is the public API; the text shape is allowed to drift.

cli/tests/marmot/tests-extras.sh

@@ -271,7 +271,7 @@ test_14_wn_removes_a() {
   local deadline=$(( $(date +%s) + 120 )) removed=0
   while [[ $(date +%s) -lt $deadline ]]; do
     local show rc
-    show=$("$AMY_BIN" --data-dir "$A_DIR" --secret-backend plaintext \
+    show=$("$AMY_BIN" --data-dir "$A_DIR" --secret-backend plaintext --json \
               marmot group show "$a_gid" 2>&1)
     rc=$?
     printf '%s\n' "$show" >>"$LOG_FILE"