fix(portability): replace bash 4 builtins absent from macOS bash 3.2

macOS ships bash 3.2 as /bin/bash, which `#!/usr/bin/env bash` resolves to
when no newer bash is on PATH. Two builtins the repo relies on do not exist
there, and both failed SILENTLY:

- `mapfile` in the three surgical-commit helpers left every array empty, so
  the scope guards passed on nothing (fail-OPEN) and the commits degraded to
  "nothing pending — no-op" while reporting success. deploy-commit.test.sh
  went 4/16; memory and doc commits simply never happened.
- `declare -A` in the session-start hook errored on every session and left
  each plugin cost at 0, so the passive-budget warning could never fire.

`_read_lines_into` is the portable equivalent of `mapfile`, space-safe and
resetting its target first — expanding a never-assigned array trips `set -u`
on bash < 4.4, which is how the empty arrays surfaced as "unbound variable".
Plugin costs move to a `case`.

source-scope.sh's header prescribed `mapfile` to its callers; it now shows
the read loop, and its own test plus run-reconcile.sh stop using the builtin.

deploy-commit 16/16, source-scope 34/34, run-reconcile 25 GREEN / 0 RED,
session-start stderr empty.

lib/memory-commit.sh

@@ -19,6 +19,19 @@ set -uo pipefail
 
 MC_PATHS=(".claude/memory" ".claude/tasks")
 
+# bash 3.2 (macOS /bin/bash) predates the `mapfile` builtin; this is the portable
+# equivalent. Reads stdin's lines into the array named by $1, space-safe
+# (IFS= read -r). The array is reset first, so empty input yields an empty array
+# rather than a stale or unset one — `set -u` on bash < 4.4 trips on expanding
+# an array that was never assigned.
+_read_lines_into() {
+  local _name="$1" _line
+  eval "$_name=()"
+  while IFS= read -r _line; do
+    eval "$_name+=(\"\$_line\")"
+  done
+}
+
 _in_git_repo() { git rev-parse --git-dir >/dev/null 2>&1; }
 
 # True (0) when the repo is in a state where we must NOT auto-commit:
@@ -59,7 +72,7 @@ commit_memory() {
     return 3
   fi
   local changed
-  mapfile -t changed < <(_changed_paths)
+  _read_lines_into changed < <(_changed_paths)
   if [ "${#changed[@]}" -eq 0 ]; then
     echo "memory-commit: nothing pending — no-op" >&2
     return 0