docs(openspec): backfill missing proposals

openspec/changes/improve-cross-platform-stability/design.md

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+## Context
+
+The stability of account injection and detection differs across platforms, with risks concentrated in:
+- DB lock conflicts causing unstable writes.
+- Version-detection failures causing incorrect injection-path routing.
+- Compatibility boundaries introduced by OS behavior differences.
+
+## Goals / Non-Goals
+
+- Goals:
+  - Improve cross-platform write and switching stability.
+  - Define clear priority between version detection and capability probing.
+  - Improve observability and troubleshooting efficiency.
+- Non-Goals:
+  - Extend support to non-default runtime configuration path parsing.
+  - Introduce new business strategies.
+
+## Decisions
+
+- Decision: Transactionalize the write flow to reduce partial-success risk.
+- Decision: Add DB busy retry and `busy_timeout`.
+- Decision: On version-detection failure, use `package.json` as Windows fallback source.
+- Decision: Give capability probing higher priority than version detection; prefer new format when a new-format key is detected.
+
+## Risks / Trade-offs
+
+- A stricter write flow may add minor latency.
+- Running capability probing and version detection together increases implementation complexity.
+
+## Migration Plan
+
+1. Complete transactional writes and retry mechanisms.
+2. Add version-detection cache and fallback.
+3. Introduce strategy-based injection and capability probing.
+4. Add corresponding tests and regression verification.