feat(coverage): pnpm coverage:aggregate + L2 implementation

Lands L2 of the agent-first coverage architecture (ADR-020) — the
aggregated trend store.

Script: scripts/coverage/aggregate.mjs (zero-dep Node ESM)
  - discoverLcovs: walks packages/* and apps/* for coverage/lcov.info
  - normalizeLcov: rewrites SF entries from package-relative (vitest's
    output) to repo-relative, so the merged file matches git diff paths
  - summarizeLcov: computes statement/branch/function/line percentages
    from LF/LH/BRF/BRH/FNF/FNH summary records
  - aggregate: merges all lcovs and returns mergedLcov + summary
  - Writes coverage/lcov.info (gitignored — large) and
    coverage/summary.json (committed — trend via git log -- ...) with
    timestamp, short commit SHA, repo + per-package percentages

Test surface: scripts/coverage/aggregate.test.mjs (10 tests, all green)
  - Fixtures at __fixtures__/aggregate-pkg-a.lcov +
    aggregate-pkg-b.lcov (synthetic, structured to make percentages
    deterministic)
  - Covers: path normalization (prefix, absolute, double-prefix
    avoidance), summary computation (percentages, zero-division,
    rounding), discovery (packages + apps, missing dirs), full
    aggregation in a tmp repo

Wired:
  - root package.json adds "coverage:aggregate" script
  - .gitignore restructured: per-package coverage/ stays ignored,
    aggregated /coverage/ ignored EXCEPT summary.json (committed for
    trend) and .gitkeep markers

L1 allowlist fix folded in (scripts/coverage/diff.mjs):
  - The previous (^|/)coverage/ regex accidentally caught
    scripts/coverage/* — replaced with anchored patterns
    (^coverage/, ^packages/*/coverage/, ^apps/*/coverage/)
  - Allowlist scripts/ and turbo/generators/ since they're dev tooling
    tested via node --test, outside vitest's v8 lcov pipeline

Smoke-tested end-to-end:
  - pnpm coverage:aggregate merged 3 lcovs (auth + media + navigation
    from this session's earlier runs), repo coverage 95.22% statements
  - pnpm coverage:diff against HEAD~1 with the new merged lcov reports
    PASS — all 6 diff files correctly allowlisted

First committed snapshot of coverage/summary.json lands with this
commit, anchoring the trend history at this state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

coverage/summary.json

@@ -0,0 +1,62 @@
+{
+  "generatedAt": "2026-05-13T12:08:33.303Z",
+  "commit": "412d994",
+  "repo": {
+    "statements": 95.22,
+    "branches": 90.52,
+    "functions": 98.77,
+    "lines": 95.22,
+    "counts": {
+      "lf": 1633,
+      "lh": 1555,
+      "brf": 232,
+      "brh": 210,
+      "fnf": 81,
+      "fnh": 80
+    }
+  },
+  "byPackage": {
+    "@repo/auth": {
+      "statements": 93.77,
+      "branches": 90.82,
+      "functions": 100,
+      "lines": 93.77,
+      "counts": {
+        "lf": 787,
+        "lh": 738,
+        "brf": 98,
+        "brh": 89,
+        "fnf": 44,
+        "fnh": 44
+      }
+    },
+    "@repo/media": {
+      "statements": 96.72,
+      "branches": 89.47,
+      "functions": 100,
+      "lines": 96.72,
+      "counts": {
+        "lf": 548,
+        "lh": 530,
+        "brf": 95,
+        "brh": 85,
+        "fnf": 24,
+        "fnh": 24
+      }
+    },
+    "@repo/navigation": {
+      "statements": 96.31,
+      "branches": 92.31,
+      "functions": 92.31,
+      "lines": 96.31,
+      "counts": {
+        "lf": 298,
+        "lh": 287,
+        "brf": 39,
+        "brh": 36,
+        "fnf": 13,
+        "fnh": 12
+      }
+    }
+  }
+}