docs: add README; adopt git.sisudoc.org/tools/ namespace

Add a top-level README.md describing what tree-sitter-sisu is, its
audience (editor / IDE tooling maintainers), its relationship to
sisudoc-spine, quickstart instructions for Neovim and Emacs 29+,
and a pointer to the existing sisu-markup_tree-sitter.md for
corpus results and grammar design notes.

Move the canonical repository URL from
https://git.sisudoc.org/software/tree-sitter-sisu to
https://git.sisudoc.org/tools/tree-sitter-sisu in both
package.json and tree-sitter.json. The new tools/ namespace
mirrors the local sisudoc-spine-tools/ directory containing this
and the other spine-companion artefacts (ssp-reader,
doc-abstraction-format), and continues the existing functional
namespace pattern on git.sisudoc.org alongside software/, markup/,
and projects/.

The README's cross-references to ssp-reader and
doc-abstraction-format use the same tools/ namespace.

(assisted by Claude-Code)

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