plan: design for SLA and escalation feature (008)

Phase 0 research resolves the business-calendar working-hours algorithm
(day-by-day walk via luxon, the first date/timezone dependency in this
codebase), the workingHours JSON shape, most-specific SLA-policy match
(reusing 005/006's resolution pattern), durable pause/resume (absolute
due-date shift, no in-memory state), and a repeatable-job breach-detection
design over per-run delayed jobs. Phase 1 adds data-model.md (one additive
refinement beyond doc06: SLARun.firstResponseBreachedAt), the admin/read
contract, and six quickstart scenarios including a genuine process-restart
boundary test for Constitution Principle VII.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package-lock.json

@@ -24,12 +24,14 @@
         "fastify": "^4.26.2",
         "fastify-plugin": "^4.5.1",
         "ioredis": "^5.3.2",
+        "luxon": "^3.7.2",
         "pino": "^8.20.0",
         "pino-pretty": "^11.0.0",
         "prom-client": "^15.1.1",
         "zod": "^3.22.4"
       },
       "devDependencies": {
+        "@types/luxon": "^3.7.5",
         "@types/node": "^20.12.7",
         "@typescript-eslint/eslint-plugin": "^7.6.0",
         "@typescript-eslint/parser": "^7.6.0",
@@ -1973,6 +1975,13 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/luxon": {
+      "version": "3.7.5",
+      "resolved": "https://registry.npmjs.org/@types/luxon/-/luxon-3.7.5.tgz",
+      "integrity": "sha512-jJ41Q4z6ZVO260MNDdHfW7+7a5iMiX8Mr6ZJHcmgrvhZha6dz5704o/lF2kKl6URjH6ivEL97w9xS/MgpJEphg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/node": {
       "version": "20.19.43",
       "resolved": "https://registry.npmjs.org/@types/node/-/node-20.19.43.tgz",