fix(#1628): type-safe config-set validation — close JSON-coercion enum bypass + enforce capability schema

Three related defects in cmdConfigSet, all 'config-set stores invalid values silently':

1. Missing guards: workflow.security_block_on (enum) and
   workflow.security_asvs_level (integer 1-3) had no store-time validation.

2. Systemic JSON-coercion bypass: every string-enum guard used
   VALID_X.includes(String(parsedValue)). Because the value is JSON-parsed
   before validation, String(["member"]) === "member" let a JSON array
   slip through and an array was stored in a scalar key. Reproduced on
   human_verify_mode, statusline.context_position, context_guard_mode,
   fallow.scope/profile, source_grounding_authority, drift_action, context.

3. Unvalidated capability keys: 32 capability-registry-owned keys (4 enum,
   25 boolean, 2 number, 1 string) had no hardcoded guard, so any value —
   including coerced arrays/objects and out-of-enum strings like
   code_review_depth=garbage — was stored silently.

Fix: a type-safe assertEnumValue() helper (requires typeof === 'string'
before membership), routed through all nine central string-enum guards
(messages preserved byte-for-byte); plus a generic capability-registry
validation block that validates every capability key against its declared
type/values (enum via the registry's values — single source of truth —
boolean, number, string). Behavioral regression tests cover every central
enum key and representative capability keys (array + object coercion
rejected, out-of-enum rejected, valid accepted) with boundary coverage for
the security keys.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/lint-test-file-count.allowlist.json

@@ -21,7 +21,8 @@
         "config-get-default.test.cjs",
         "config-schema.property.test.cjs",
         "config.test.cjs",
-        "enh-1055-config-intent-descriptor-drive.test.cjs"
+        "enh-1055-config-intent-descriptor-drive.test.cjs",
+        "fix-1628-config-set-validation.test.cjs"
       ],
       "issue": "TBD"
     },