Move grund.toml to the project roots

[vjovanov]

Closes #1025.

grund probes a bare grund.toml before .agents/grund.toml, so all four configs move up to the project root each one configures — the workspace root plus common, native-gradle-plugin, and native-maven-plugin.

What's in here

1. Four git mvs, pure renames. No config keys change.
2. .github/workflows/check-grund-spec.yml: path filter .agents/grund.toml → grund.toml, and GRUND_VERSION 0.5.0 → 0.9.0. The member configs stay covered by the existing common/** and native-*-plugin/** filters.
3. grund init in each of the four projects, refreshing the managed block in every AGENTS.md from v3 to v7.

Why the version bump is load-bearing

Root discovery landed in grund 0.9.0. Pinned at 0.5.0 the gate would look only under .agents/, find nothing, and exit 0 without checking anything — green CI validating an empty set. I confirmed that zero-config behavior directly: grund check on a tree holding a declaration and a dangling citation still prints success and exits 0.

Pre-existing failure this surfaces

The bump is not free. Under 0.9.0 all four AGENTS.md files already fail grund check on outdated grund init block v3, independently of this move — which is why the block refresh is in the same PR rather than a follow-up. Beyond the config-path sentence, v7 adds the BEGIN/END block markers, a "Marker = live citation" rule, and a clickable-citations paragraph.

Verification

With grund 0.9.0 locally, both gate commands pass: grund check and grund fmt . --marker --cross-refs --check exit 0. Before the change, grund check exited 1 with the four v3 errors.

[jvukicev]

LGTM