v0.3.5

Add local-checkout skill symlink installation (#304)

## Summary

- add opt-in `AMUX_SKILLS_SOURCE` support to the shell installer
- link `/amux`, `/amux-claude`, and `/amux-pi` from one canonical local
checkout under `~/.agents/skills`
- preserve replaced real installs and same-name legacy duplicates as
timestamped backups
- reject invalid sources, unsafe destination parents, source/destination
overlap, and backup collisions before mutation
- keep binary-only installation unchanged when the option is absent
- document explicit checkout refresh and Amp reload behavior

## Verification

- `/bin/sh -n docs/install.sh`
- `go test ./scripts -count=1`
- `go test ./...`
- `go vet ./...`
- `go build ./cmd/amux`
- `git diff --check`