Create PLUGIN_SCHEMA_NOTES.md

This PR documents and stabilizes the Claude Code plugin manifest schema based on real validator behavior observed in Issue #90.

.claude-plugin/PLUGIN_SCHEMA_NOTES.md

@@ -0,0 +1,180 @@
+# Plugin Manifest Schema Notes
+
+This document captures **undocumented but enforced constraints** of the Claude Code plugin manifest validator.
+
+These rules are based on real installation failures, validator behavior, and comparison with known working plugins.
+They exist to prevent silent breakage and repeated regressions.
+
+If you edit `.claude-plugin/plugin.json`, read this first.
+
+---
+
+## Summary (Read This First)
+
+The Claude plugin manifest validator is **strict and opinionated**.
+It enforces rules that are not fully documented in public schema references.
+
+The most common failure mode is:
+
+> The manifest looks reasonable, but the validator rejects it with vague errors like
+> `agents: Invalid input`
+
+This document explains why.
+
+---
+
+## Required Fields
+
+### `version` (MANDATORY)
+
+The `version` field is required by the validator even if omitted from some examples.
+
+If missing, installation may fail during marketplace install or CLI validation.
+
+Example:
+
+```json
+{
+  "version": "1.1.0"
+}
+```
+
+---
+
+## Field Shape Rules
+
+The following fields **must always be arrays**:
+
+* `agents`
+* `commands`
+* `skills`
+* `hooks` (if present)
+
+Even if there is only one entry, **strings are not accepted**.
+
+### Invalid
+
+```json
+{
+  "agents": "./agents"
+}
+```
+
+### Valid
+
+```json
+{
+  "agents": ["./agents/planner.md"]
+}
+```
+
+This applies consistently across all component path fields.
+
+---
+
+## Path Resolution Rules (Critical)
+
+### Agents MUST use explicit file paths
+
+The validator **does not accept directory paths for `agents`**.
+
+Even the following will fail:
+
+```json
+{
+  "agents": ["./agents/"]
+}
+```
+
+Instead, you must enumerate agent files explicitly:
+
+```json
+{
+  "agents": [
+    "./agents/planner.md",
+    "./agents/architect.md",
+    "./agents/code-reviewer.md"
+  ]
+}
+```
+
+This is the most common source of validation errors.
+
+### Commands and Skills
+
+* `commands` and `skills` accept directory paths **only when wrapped in arrays**
+* Explicit file paths are safest and most future-proof
+
+---
+
+## Validator Behavior Notes
+
+* `claude plugin validate` is stricter than some marketplace previews
+* Validation may pass locally but fail during install if paths are ambiguous
+* Errors are often generic (`Invalid input`) and do not indicate root cause
+* Cross-platform installs (especially Windows) are less forgiving of path assumptions
+
+Assume the validator is hostile and literal.
+
+---
+
+## Known Anti-Patterns
+
+These look correct but are rejected:
+
+* String values instead of arrays
+* Arrays of directories for `agents`
+* Missing `version`
+* Relying on inferred paths
+* Assuming marketplace behavior matches local validation
+
+Avoid cleverness. Be explicit.
+
+---
+
+## Minimal Known-Good Example
+
+```json
+{
+  "version": "1.1.0",
+  "agents": [
+    "./agents/planner.md",
+    "./agents/code-reviewer.md"
+  ],
+  "commands": ["./commands/"],
+  "skills": ["./skills/"]
+}
+```
+
+This structure has been validated against the Claude plugin validator.
+
+---
+
+## Recommendation for Contributors
+
+Before submitting changes that touch `plugin.json`:
+
+1. Use explicit file paths for agents
+2. Ensure all component fields are arrays
+3. Include a `version`
+4. Run:
+
+```bash
+claude plugin validate .claude-plugin/plugin.json
+```
+
+If in doubt, choose verbosity over convenience.
+
+---
+
+## Why This File Exists
+
+This repository is widely forked and used as a reference implementation.
+
+Documenting validator quirks here:
+
+* Prevents repeated issues
+* Reduces contributor frustration
+* Preserves plugin stability as the ecosystem evolves
+
+If the validator changes, update this document first.