docs: add troubleshooting section to README

[MacAttak]

Context

Users hitting common issues (e.g. plugin not loading, compaction recovery, state file corruption) currently have no troubleshooting guide.

Suggested scope

• Add a "Troubleshooting" section to the README (or a separate docs/troubleshooting.md)
• Cover common issues: plugin installation problems, /sw-status --reset for stuck state, compaction recovery
• Reference protocols/recovery.md for compaction details

Helpful references

• protocols/recovery.md — compaction recovery procedure
• skills/sw-status/SKILL.md — describes --reset flag
• CLAUDE.md — key rules section