docs(bin): give every bin skill a README

The bin skills were the only plugin without per-skill READMEs, so a reader
had to open SKILL.md — an agent-facing contract, not an explainer — to learn
what a skill does and when it fires. Each README states purpose, triggers and
boundaries for a human audience, leaving SKILL.md free to stay terse.

plugins/bin/.apm/skills/diagnose/README.md

@@ -0,0 +1,35 @@
+# diagnose
+
+A six-phase discipline for hard bugs and performance regressions: feedback loop → reproduce → hypothesise → instrument → fix with a regression test → clean up.
+
+## What it does
+
+Imposes an order of operations on debugging so the agent cannot skip to guessing. The load-bearing phase is the first one: build a fast, deterministic, agent-runnable pass/fail signal for the bug. Everything downstream — bisection, hypothesis testing, instrumentation — just consumes that signal, so the skill refuses to advance to Phase 2 without one, and says so explicitly rather than hypothesising blind.
+
+The remaining phases each carry a constraint worth knowing about: hypotheses are generated 3–5 at a time and must be falsifiable, so the first plausible idea cannot anchor the whole investigation; every debug log is tagged with a unique prefix (`[DEBUG-a4f2]`) so cleanup is a single grep; the regression test is written before the fix and only at a seam that exercises the real bug pattern; and the run closes by asking what would have prevented the bug, handing off to `improve-codebase-architecture` when the answer is architectural.
+
+Performance regressions take a branch of their own inside Phase 4 — baseline measurement and bisection, not logs.
+
+## Conditional reading
+
+Neither reference file is read on every run; `SKILL.md` names the condition for each.
+
+- `references/feedback-loops.md` is read when Phase 1 has no signal yet, or when the loop you have is slow or intermittent.
+- `references/regression-seams.md` is read when Phase 5 leaves you unsure whether the available seam is deep enough — or whether one exists at all.
+
+## Usage
+
+```text
+/diagnose
+```
+
+Describe the bug or the regression. For filing and triaging a reported bug rather than diagnosing it, use `triage`; for test-first feature work, use `tdd`.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `SKILL.md` | The six phases and their gates — what must be true before each one ends |
+| `references/feedback-loops.md` | Loaded when Phase 1 has no loop or the loop is too weak: ten ways to construct one ordered by cost, how to sharpen an existing loop, handling intermittent bugs, and what to ask the user for when the bug resists reproduction |
+| `references/regression-seams.md` | Loaded when Phase 5 is unsure about the seam: what makes a seam correct, the four shapes of a too-shallow seam, and what to do when no correct seam exists |
+| `assets/hitl-loop.template.sh` | Copy-and-edit bash template for the last-resort human-in-the-loop feedback loop, cited by `references/feedback-loops.md`. Provides `step` and `capture` helpers and prints captured values as `KEY=VALUE` for the agent to parse |