feat(whatsapp): redesign ack-reaction as whatsapp-specific feature

- Move config from messages.ackReaction to whatsapp.ackReaction
- New structure: {emoji, direct, group} with granular control
- Support per-account overrides in whatsapp.accounts.*.ackReaction
- Add Zod schema validation for new config
- Maintain backward compatibility with old messages.ackReaction format
- Update tests to new config structure (14 tests, all passing)
- Add comprehensive documentation in docs/providers/whatsapp.md
- Timing: reactions sent immediately upon message receipt (before bot reply)

Breaking changes:
- Config moved from messages.ackReaction to whatsapp.ackReaction
- Scope values changed: 'all'/'direct'/'group-all'/'group-mentions'
  → direct: boolean + group: 'always'/'mentions'/'never'
- Old config still supported via fallback for smooth migration

src/config/zod-schema.ts

@@ -1375,6 +1375,16 @@ export const ClawdbotSchema = z
                       .optional(),
                   )
                   .optional(),
+                ackReaction: z
+                  .object({
+                    emoji: z.string().optional(),
+                    direct: z.boolean().optional().default(true),
+                    group: z
+                      .enum(["always", "mentions", "never"])
+                      .optional()
+                      .default("mentions"),
+                  })
+                  .optional(),
               })
               .superRefine((value, ctx) => {
                 if (value.dmPolicy !== "open") return;
@@ -1421,6 +1431,16 @@ export const ClawdbotSchema = z
               .optional(),
           )
           .optional(),
+        ackReaction: z
+          .object({
+            emoji: z.string().optional(),
+            direct: z.boolean().optional().default(true),
+            group: z
+              .enum(["always", "mentions", "never"])
+              .optional()
+              .default("mentions"),
+          })
+          .optional(),
       })
       .superRefine((value, ctx) => {
         if (value.dmPolicy !== "open") return;