refactor(plugin)!: ship as a Claude Code plugin, one dir link per harness

Skills move to skills/<name>/SKILL.md and the repo gains both manifests, so it is now a marketplace hosting exactly one plugin whose root is the repo root (source: "."). Claude Code installs it from git; no symlinks involved. Why: install.sh wired the library into every consuming repo as an ABSOLUTE symlink to $CHECKOUT_DIR. The checkout moved three times, most recently when local-dev@1966d5a dropped the embedded .skills/ copy, and nothing migrated the links — 243 dangling links across 12 repos, hidden from git by the **/.claude/skills/ ignore rule. Worse, ~/dev spans two path roots (/Users/mathias on the iCloud-shared Macs, /home/mathias on koala), so an absolute link stored there is broken on one side by construction. Re-running the installer cannot fix that; only removing the mechanism can. install.sh keeps the harnesses with no plugin system and drops from 42 links per harness to one directory link, via the convergent ~/.agents/skills convention that Pi and OpenCode both read: ~/.agents/skills -> $CHECKOUT_DIR/skills ~/.config/crush/skills -> ~/.agents/skills ~/.config/opencode/skills -> ~/.agents/skills wire_claude_repo is deleted outright rather than gated behind a flag — on a mixed-root fleet it cannot be made correct. Antigravity and Vibe targets go too; both unused. CHECKOUT_DIR keeps its host-local default deliberately: pointing it into ~/dev would put a git working tree inside iCloud with two concurrent writers. Taskfile drops five near-identical wiring loops (they had already drifted) and delegates to install.sh. `task check` gains frontmatter validation, a stray-root-entry gate, and manifest/tag version parity — the loose secret-hygiene.md sat unloadable for months because nothing checked layout. Verified: claude plugin validate . passes clean; a probe skill loaded as skills:zzprobe through --plugin-dir; task check green at 42 skills. Refs: #23 Bump-Type: minor Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01BeAp5LTscnz2W7Ubt4eJ6m
2026-08-23 14:29:51 +02:00
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+# Defense-in-Depth Validation
+
+## Overview
+
+When you fix a bug caused by invalid data, adding validation at one place feels sufficient. But that single check can be bypassed by different code paths, refactoring, or mocks.
+
+**Core principle:** Validate at EVERY layer data passes through. Make the bug structurally impossible.
+
+## Why Multiple Layers
+
+Single validation: "We fixed the bug"
+Multiple layers: "We made the bug impossible"
+
+Different layers catch different cases:
+- Entry validation catches most bugs
+- Business logic catches edge cases
+- Environment guards prevent context-specific dangers
+- Debug logging helps when other layers fail
+
+## The Four Layers
+
+### Layer 1: Entry Point Validation
+**Purpose:** Reject obviously invalid input at API boundary
+
+```typescript
+function createProject(name: string, workingDirectory: string) {
+  if (!workingDirectory || workingDirectory.trim() === '') {
+    throw new Error('workingDirectory cannot be empty');
+  }
+  if (!existsSync(workingDirectory)) {
+    throw new Error(`workingDirectory does not exist: ${workingDirectory}`);
+  }
+  if (!statSync(workingDirectory).isDirectory()) {
+    throw new Error(`workingDirectory is not a directory: ${workingDirectory}`);
+  }
+  // ... proceed
+}
+```
+
+### Layer 2: Business Logic Validation
+**Purpose:** Ensure data makes sense for this operation
+
+```typescript
+function initializeWorkspace(projectDir: string, sessionId: string) {
+  if (!projectDir) {
+    throw new Error('projectDir required for workspace initialization');
+  }
+  // ... proceed
+}
+```
+
+### Layer 3: Environment Guards
+**Purpose:** Prevent dangerous operations in specific contexts
+
+```typescript
+async function gitInit(directory: string) {
+  // In tests, refuse git init outside temp directories
+  if (process.env.NODE_ENV === 'test') {
+    const normalized = normalize(resolve(directory));
+    const tmpDir = normalize(resolve(tmpdir()));
+
+    if (!normalized.startsWith(tmpDir)) {
+      throw new Error(
+        `Refusing git init outside temp dir during tests: ${directory}`
+      );
+    }
+  }
+  // ... proceed
+}
+```
+
+### Layer 4: Debug Instrumentation
+**Purpose:** Capture context for forensics
+
+```typescript
+async function gitInit(directory: string) {
+  const stack = new Error().stack;
+  logger.debug('About to git init', {
+    directory,
+    cwd: process.cwd(),
+    stack,
+  });
+  // ... proceed
+}
+```
+
+## Applying the Pattern
+
+When you find a bug:
+
+1. **Trace the data flow** - Where does bad value originate? Where used?
+2. **Map all checkpoints** - List every point data passes through
+3. **Add validation at each layer** - Entry, business, environment, debug
+4. **Test each layer** - Try to bypass layer 1, verify layer 2 catches it
+
+## Example from Session
+
+Bug: Empty `projectDir` caused `git init` in source code
+
+**Data flow:**
+1. Test setup → empty string
+2. `Project.create(name, '')`
+3. `WorkspaceManager.createWorkspace('')`
+4. `git init` runs in `process.cwd()`
+
+**Four layers added:**
+- Layer 1: `Project.create()` validates not empty/exists/writable
+- Layer 2: `WorkspaceManager` validates projectDir not empty
+- Layer 3: `WorktreeManager` refuses git init outside tmpdir in tests
+- Layer 4: Stack trace logging before git init
+
+**Result:** All 1847 tests passed, bug impossible to reproduce
+
+## Key Insight
+
+All four layers were necessary. During testing, each layer caught bugs the others missed:
+- Different code paths bypassed entry validation
+- Mocks bypassed business logic checks
+- Edge cases on different platforms needed environment guards
+- Debug logging identified structural misuse
+
+**Don't stop at one validation point.** Add checks at every layer.