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/cache/cache-headless.sh

@@ -81,7 +81,7 @@ source "$TESTS_DIR/headless/helpers.sh"
 # shellcheck source=../dm/setup.sh
 source "$TESTS_DIR/dm/setup.sh"
 
-amy_b() { "$AMY_BIN" --data-dir "$B_DIR" "$@"; }
+amy_b() { "$AMY_BIN" --data-dir "$B_DIR" --json "$@"; }
 
 # Helper: B-side amy_json. The dm headless's helpers.sh hardcodes A_DIR
 # in amy_a; we need a parallel for B without re-sourcing.
@@ -249,10 +249,10 @@ fi
 # ----------------------------------------------------------------------
 banner "T7 — store maintenance verbs (sweep/scrub/compact) without identity"
 TMP_NO_ID=$(mktemp -d)
-T7_STAT=$(${AMY_BIN} --data-dir "$TMP_NO_ID" store stat)
-T7_SWEEP=$(${AMY_BIN} --data-dir "$TMP_NO_ID" store sweep-expired)
-T7_SCRUB=$(${AMY_BIN} --data-dir "$TMP_NO_ID" store scrub)
-T7_COMPACT=$(${AMY_BIN} --data-dir "$TMP_NO_ID" store compact)
+T7_STAT=$(${AMY_BIN} --data-dir "$TMP_NO_ID" --json store stat)
+T7_SWEEP=$(${AMY_BIN} --data-dir "$TMP_NO_ID" --json store sweep-expired)
+T7_SCRUB=$(${AMY_BIN} --data-dir "$TMP_NO_ID" --json store scrub)
+T7_COMPACT=$(${AMY_BIN} --data-dir "$TMP_NO_ID" --json store compact)
 assert_eq "$(printf '%s' "$T7_STAT" | jq -r '.events')" "0" T7.stat.events "" \
     && record_result T7.stat pass "stat works without identity"
 assert_eq "$(printf '%s' "$T7_SWEEP" | jq -r '.swept')" "0" T7.sweep.swept "" \