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/dm/setup.sh

@@ -75,15 +75,15 @@ preflight_dm() {
 # `--secret-backend=plaintext` keeps these throwaway interop runs headless —
 # the default `auto` would try the OS keychain (not available in CI) and then
 # ask for a NIP-49 passphrase. Plaintext still writes 0600-owner-only.
-amy_a() { "$AMY_BIN" --data-dir "$A_DIR" --secret-backend plaintext "$@"; }
-amy_d() { "$AMY_BIN" --data-dir "$D_DIR" --secret-backend plaintext "$@"; }
+amy_a() { "$AMY_BIN" --data-dir "$A_DIR" --secret-backend plaintext --json "$@"; }
+amy_d() { "$AMY_BIN" --data-dir "$D_DIR" --secret-backend plaintext --json "$@"; }
 
 # --- identity bootstrap ------------------------------------------------------
 ensure_identity_for() {
   local who="$1" dir="$2"
   step "initialising Identity $who (amy at $dir)"
   local out
-  out=$("$AMY_BIN" --data-dir "$dir" --secret-backend plaintext init) || {
+  out=$("$AMY_BIN" --data-dir "$dir" --secret-backend plaintext --json init) || {
     fail_msg "amy init failed for $who: $out"; exit 1
   }
   local npub hex