fix(plugins): replace mapfile with while-read for bash 3.2 compat (#3636)

`mapfile` is a bash 4.0 builtin and is unavailable on macOS's system
bash 3.2.57. Commit 4d63c18 ("fix(plugins): auto-discover databases
instead of hardcoded list") introduced `mapfile -t PROD_DBS < <(...)`
in both dolt-archive/run.sh and dolt-backup/run.sh. On macOS, every
patrol invocation of these plugins exits 127 with:

    line 69: mapfile: command not found   (dolt-archive)
    line 49: mapfile: command not found   (dolt-backup)

Impact: JSONL last-resort recovery layer (dolt-archive) and rsync-style
Dolt backup (dolt-backup) silently fail on macOS. Both plugins are the
operational safety net for beads databases.

The bug is gated by the auto-discovery branch — passing explicit
`--databases foo,bar` takes the IFS/read path and still works, which
likely explains why the regression went unnoticed in manual testing.
The default `DEFAULT_DBS="auto"` triggers the break for all scheduled
runs.

Fix: replace `mapfile -t ARR < <(...)` with the bash 3.2-compatible
`ARR=(); while IFS= read -r line; do ARR+=("$line"); done < <(...)`
pattern. Same fix was previously applied in commits e297e73 and
4c5f269 and subsequently regressed by 4d63c18.

Verified on darwin25 / bash 3.2.57(1)-release:
  - plugins/dolt-archive/run.sh auto-discovers 16 production DBs
  - plugins/dolt-backup/run.sh --dry-run auto-discovers 16 production DBs
  - Both scripts run to completion with exit 0

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kingb

plugins/dolt-archive/run.sh

@@ -66,7 +66,10 @@ dolt_query_json() {
 
 # Auto-discover production databases or use the explicit list.
 if [[ "$DEFAULT_DBS" == "auto" ]]; then
-  mapfile -t PROD_DBS < <(
+  PROD_DBS=()
+  while IFS= read -r line; do
+    PROD_DBS+=("$line")
+  done < <(
     dolt_query "" "SHOW DATABASES" \
       | grep -v -E '^(information_schema|mysql|dolt_cluster)$' \
       | grep -v -E '^(testdb_|beads_t|beads_pt|doctest_)'

plugins/dolt-backup/run.sh

@@ -46,7 +46,10 @@ log() {
 if [[ -n "$EXPLICIT_DBS" ]]; then
   IFS=',' read -ra PROD_DBS <<< "$EXPLICIT_DBS"
 else
-  mapfile -t PROD_DBS < <(
+  PROD_DBS=()
+  while IFS= read -r line; do
+    PROD_DBS+=("$line")
+  done < <(
     for d in "$DOLT_DATA_DIR"/*/; do
       name="$(basename "$d")"
       [[ -d "$d/.dolt" ]] || continue