docs: bug-report template asks for doctor output; README troubleshooting entry; doctor test in the Windows lane

The template gives the Claude Code doctor path and a lookup for other
hosts and names no bare gstack- command (asserted in
test/gstack-doctor.test.ts). The doctor test runs every script through
explicit bash argv, so it joins the curated Windows free subset.

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,40 @@
+---
+name: Bug report
+about: A gstack skill, helper or setup step broke or did something unexpected
+labels: bug
+---
+
+## What happened
+
+<!-- The skill or command you ran, what it printed, and what you expected instead. -->
+
+## Steps to reproduce
+
+1.
+2.
+
+## Readiness report
+
+Paste the complete output of gstack's doctor. It starts no skill, makes no paid
+call and prints no secrets or file contents. Run the line for your install:
+
+```bash
+# Claude Code
+~/.claude/skills/gstack/bin/gstack-doctor
+
+# Any other host: from your gstack checkout, ./setup --status ends with the
+# absolute path of the doctor ("Readiness check: ..."). Run that path.
+./setup --status
+
+# The state root (where installs.tsv lists every install) for the same checkout:
+./bin/gstack-paths --get GSTACK_STATE_ROOT
+```
+
+```text
+(doctor output here)
+```
+
+## Environment
+
+- Host (Claude Code, Codex, Cursor, ...) and its version:
+- OS: