mara on PR #2769: "make the default null instead of special casing """.
`claude_code_path` was a `String` whose empty value meant "no host-level
pin". That is a sentinel doing an `Option`'s job — the same shape argus
and mara already rejected on #2755's weights, and the same
empty-field cruft mara called out on #2756.
So it is `Option<String>` end to end:
- host module: `claudeCodePath` evaluates to `null` when
`claudeCodePackage` is unset, so `serve.json` carries JSON `null`
rather than `""`.
- `Coordinator` + `HiveEnv`: `Option<String>`, defaulting to `None`.
- `render_flake`/`render_flake_with_lookup`: `Option<&str>`, and the
emission is an `if let Some(path)` instead of an `is_empty()` guard.
- agent module: `hyperhive.claudeCodePath` is `nullOr str`, default
`null`.
Behaviour is unchanged in both directions; only the way "unset" is
spelled moves. The `builtins.hasContext` assertion still guards the
pinned case (short-circuited by the null check, so an unpinned hive
never evaluates it).
16/16 `meta::` tests, clippy clean, `nix fmt` no-op, `nix build .#docs`
green.
2ad4b43118