docs: add GitHub issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Something steplock does incorrectly
+labels: bug
+---
+
+**steplock version** (`steplock --version`):
+
+**OS / platform**:
+
+**Rust version** (`rustc --version`):
+
+**Checklist config** (`config.toml` and `flow.mmd`):
+
+```toml
+# config.toml
+```
+
+```
+# flow.mmd
+```
+
+**Command that triggered the hook**:
+
+**Expected behaviour**:
+
+**Actual behaviour** (paste the full hook output):
+
+```
+```
+
+**Session state** (`cat .steplock/sessions/<id>/state.json`), if relevant:
+
+```json
+```

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Propose a new capability for steplock
+labels: enhancement
+---
+
+**What problem does this solve?**
+
+**Proposed solution**:
+
+**Alternatives considered**:
+
+**Additional context**: