Add architecture review findings and address documentation issues

Review of all ADR documents (001-007) and peripheral architecture docs
identified 3 critical, 10 warning, and 7 suggestion issues.

Addressed in this commit:
- W-1: Add draft qualifier to ADR-002 reference to incremental exploration
- W-2: Add Alternatives Considered section to ADR-001
- W-3: Add Document Lifecycle section to README.md (draft/stable/deprecated)
- W-4: Clarify includeCompleted semantics (only 'completed' status triggers exclusion)
- W-5: Document file I/O runtime constraints in frontmatter.md
- W-6: Add ADR reference to architecture.md redirect
- W-7: Verify CVE-2025-64718 (confirmed real, improved description)
- W-9: Convert workspace-absolute paths to relative/monorepo references
- S-7: Add future ADR-008 note to incremental-update-exploration.md

Critical issues (C-1, C-2, C-3) and remaining warnings (W-8, W-10, S-4, S-5)
were addressed by a parallel agent in a prior commit.

All 16 review tasks created and resolved.

tasks/architecture/c-3-nullable-helper.md

@@ -0,0 +1,21 @@
+---
+id: architecture/c-3-nullable-helper
+name: Move Nullable helper definition before first use in schemas.md
+status: completed
+depends_on: []
+created: 2026-04-26T09:10:33.686014464Z
+modified: 2026-04-26T09:10:33.686014969Z
+scope: narrow
+risk: high
+---
+
+# Description
+
+**Review ref**: C-3 (Critical)
+**Files affected**: `docs/architecture/schemas.md`
+
+The `Nullable` helper is used extensively in the `TaskInput` schema (lines 54–63) but only defined at line 219 in the `ResolvedTaskAttributes` section. An implementer reading top-down encounters `Nullable` without understanding what it does. Also unclear whether `Nullable` is from `@alkdev/typebox` or defined locally.
+
+Add a "Shared Schema Utilities" section near the top of `schemas.md` (before `TaskInput`) that defines `Nullable` and its behavior. Or at minimum add a forward reference: "see Shared Schema Utilities below for the `Nullable` helper" at first use.
+
+**Source**: `/docs/reviews/architecture-review-2026-04-26.md` C-3