v3.1.0

docs: refresh CLI and contributor guidance (#956)

## Summary

- align the root README and contributor guide with the current
repository layout, Rust toolchain, CLI command surface, storage
architecture, and local image
- update CLI docs from stale v2 terminology to the current v3 model
- document `/healthz` as the local startup readiness contract while
retaining `/v2/query` as the query endpoint
- regenerate `docs/llms.txt` and `docs/llms-full.txt`

## Notable cleanup

- remove a broken root README link
- fix the CLI install path and source-tree paths
- remove a nonexistent `helix dashboard` command
- remove obsolete LMDB and unsupported comparative performance claims
- add the missing `helix skills` command

## Validation

- `cargo test -p helix-cli --doc`
- `cargo fmt --all -- --check`
- `npm run check` in `docs/`
- repository-wide stale image/runtime terminology scan
- verified root external documentation/install links return HTTP 200