feat: add diagnose plugin

.claude-plugin/marketplace.json

@@ -66,6 +66,12 @@
       "source": "./plugins/bug-report",
       "category": "workflow",
       "tags": ["bug", "bug-report", "defect", "triage", "issue", "skill"]
+    },
+    {
+      "name": "diagnose",
+      "source": "./plugins/diagnose",
+      "category": "debugging",
+      "tags": ["debug", "diagnose", "troubleshooting", "root-cause", "regression", "skill"]
     }
   ]
 }

README.md

@@ -30,6 +30,7 @@ Browse and manage with `/plugin`. To refresh after updates: `/plugin marketplace
 | [`actionlint`](plugins/actionlint) | Hook | Lints GitHub Actions workflow files (`.github/workflows/*.yml`/`.yaml`) on edit via the `actionlint` already on your `PATH` — advisory findings, never blocking. |
 | [`guardrails`](plugins/guardrails) | Hook | Bundles four independently-toggleable PreToolUse safety guards: secret-pattern detection, hardcoded machine-path check, git hook-bypass blocking (`--no-verify`, `core.hooksPath`, `LEFTHOOK=0`), and advisory CLI-flag verification. |
 | [`bug-report`](plugins/bug-report) | Skill | Turns an informal defect description into a structured five-field bug report (title, repro, expected vs actual, severity, fix location). Read-only — it captures, never fixes or files on its own. |
+| [`diagnose`](plugins/diagnose) | Skill | Debugs observed failures via a disciplined six-phase loop — build a fast deterministic reproduction signal, reproduce, rank falsifiable hypotheses, instrument, fix with a regression test, then clean up and post-mortem. |
 
 Install one: `/plugin install <plugin-name>@melodic-software`.
 

plugins/diagnose/.claude-plugin/plugin.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://json.schemastore.org/claude-code-plugin-manifest.json",
+  "name": "diagnose",
+  "version": "0.1.0",
+  "description": "Debug observed failures via a disciplined six-phase loop: build a fast deterministic reproduction signal, reproduce, rank falsifiable hypotheses, instrument, fix with a regression test, then clean up and post-mortem.",
+  "author": {
+    "name": "Melodic Software",
+    "email": "info@melodicsoftware.com"
+  },
+  "keywords": ["debug", "diagnose", "debugging", "troubleshooting", "root-cause", "regression", "skill"]
+}

plugins/diagnose/README.md

@@ -0,0 +1,56 @@
+# diagnose
+
+A Claude Code plugin that debugs **observed failures** — a wrong UI, a bad log
+line, a performance regression, a screenshot of a bug, a production symptom —
+via a disciplined six-phase loop. It is the discipline that separates a fixed
+bug from a lucky one: no phase proceeds without a fast, deterministic,
+agent-runnable pass/fail signal.
+
+Invoke it with `/diagnose:diagnose <bug description>`, or let Claude reach for it
+when you describe broken behavior with no pre-existing reproduction.
+
+## The six phases
+
+1. **Build a tight feedback loop** — the load-bearing work. A fast, deterministic
+   signal that says "bug present / bug fixed". Ten construction strategies, from a
+   failing test to a human-in-the-loop script.
+2. **Reproduce** — run the loop; confirm it shows *the* failure the user described.
+3. **Hypothesise** — 3-5 ranked, falsifiable hypotheses before testing any.
+4. **Instrument** — one probe per prediction, one variable at a time; tagged debug
+   logs that clean up with a single grep. A dedicated performance branch.
+5. **Fix + regression test** — test at a *correct seam* first; if none exists, that
+   absence is itself the finding.
+6. **Cleanup + post-mortem** — remove instrumentation, verify the original repro is
+   gone, and capture what would have prevented the bug.
+
+## Works in any repo
+
+- **Self-contained.** The methodology, the per-ecosystem debugging reference, the
+  phase checklist, and the human-in-the-loop script template all ship inside the
+  plugin and are referenced via `${CLAUDE_PLUGIN_ROOT}`.
+- **Graceful degrade.** Where a phase mentions an adjacent capability — a
+  test-investigation routine, a TDD helper, a headless-browser driver, an
+  architecture-audit agent, an issue tracker, an outcome verifier — it is treated as
+  **optional**: if your environment provides it, the skill uses it; otherwise it
+  proceeds with self-contained inline guidance. No phase blocks on a missing tool.
+- **Reads your conventions, assumes none.** Test naming, module layout, banned APIs,
+  and where working notes live come from your own project's `CLAUDE.md` /
+  `.claude/rules` and tool config.
+
+## Install
+
+```shell
+/plugin marketplace add melodic-software/claude-code-plugins
+/plugin install diagnose@melodic-software
+```
+
+## Configuration
+
+This plugin has no `userConfig`. The phase checklist is a bundled template you copy
+into your own working-notes location (or track inline); nothing is written to shared
+plugin storage.
+
+## License
+
+MIT (SPDX-License-Identifier: MIT). See the `LICENSE` file at the root of the
+melodic-software/claude-code-plugins repository.