ci: make the bracket-assign guard actually match its targets

The prototype-pollution rule has never fired. Its `paths.include` named the
pre-workspace layout (`/src/providers/*.ts`, `/src/parser.ts`) while the job
scans `packages/cli/src/...`, so the rule selected no files and the step was
green unconditionally.

Verified before and after: with the old rule and the old scan targets a
planted bracket-assign under packages/cli/src/providers/ reports 0 findings;
with this change the same violation is reported at the right file and line,
and the real tree scans 184 files clean.

Also brings packages/core/src/providers/** into scope. Provider decoding moved
there in the extraction, so that is where records from untrusted session logs
are now turned into maps — precisely what the rule exists to guard.

Two guard-rails so the step cannot silently go vacuous again:

- The step now fails when the scan selected no files. semgrep's --json
  `paths.scanned` reflects what the rule actually selected, not the raw scan
  targets (verified: with the include paths broken but the targets intact,
  scanned drops to 0 while the step still exits 0). A low-water mark was
  considered and rejected as brittle: the provider count is a moving target
  by design and CI installs semgrep unpinned, so a magic number would need
  constant bumping and would decay back into vacuity.

- The CLI providers glob is now recursive (`**/*.ts`) like core's. The
  directory is flat today, but a future nested provider directory would be
  scanned by CI and then silently excluded by the rule; a planted violation
  under providers/nested/ is now reported. The narrow whitelist is
  deliberate and unchanged in spirit: unrelated maps elsewhere in the tree
  stay out of scope.

.github/workflows/ci.yml

@@ -54,7 +54,12 @@ jobs:
           set -e
           semgrep --config .semgrep/rules/no-bracket-assign-hot-paths.yml \
             --strict --json \
-            packages/cli/src/providers/ packages/cli/src/parser.ts > semgrep-out.json
+            packages/cli/src/providers/ packages/cli/src/parser.ts packages/core/src/providers/ > semgrep-out.json
+          SCANNED=$(jq '.paths.scanned | length' semgrep-out.json)
+          if [ "$SCANNED" -eq 0 ]; then
+            echo "::error::semgrep scanned no files — the rule's paths.include no longer matches any scan target; check .semgrep/rules/no-bracket-assign-hot-paths.yml"
+            exit 1
+          fi
           FINDINGS=$(jq '.results | length' semgrep-out.json)
           if [ "$FINDINGS" -gt 0 ]; then
             jq -r '.results[] | "::error file=\(.path),line=\(.start.line)::\(.extra.message)"' semgrep-out.json

.semgrep/rules/no-bracket-assign-hot-paths.yml

@@ -18,5 +18,6 @@ rules:
           ...
     paths:
       include:
-        - '/src/providers/*.ts'
-        - '/src/parser.ts'
+        - '/packages/cli/src/providers/**/*.ts'
+        - '/packages/cli/src/parser.ts'
+        - '/packages/core/src/providers/**/*.ts'