add ADR-0023 (proposed): unified declarative grammar tree

Captures the architectural critique surfaced during round-5
manual testing — that adding a keyword or command currently
requires edits in 7-10 files across parser, completion, usage
registry, catalog, and tests — and the proposed direction: a
single declarative trie registry that drives parse, completion,
highlight, and usage rendering from one source.

Status: Proposed. Not yet accepted. Filename carries the
`-proposed-` segment so status is visible at directory-listing
time; rename to `0023-unified-grammar-tree.md` on acceptance.

Estimated cost: ~4 sessions, per-command migration. Why not
now: feature backlog and bearable scatter cost. Right moment
to execute when backlog quiets or scatter cost becomes
visibly painful.

docs/adr/README.md

@@ -28,3 +28,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0020 — Tokenization layer for the DSL parser](0020-tokenization-layer-for-the-dsl-parser.md)
 - [ADR-0021 — Parser-as-source-of-truth for H1a (per-command usage in parse errors)](0021-parser-as-source-of-truth-for-h1a.md)
 - [ADR-0022 — Ambient typing assistance: colour, hint panel, completion (I3 + I4)](0022-ambient-typing-assistance.md)
+- [ADR-0023 — Unified declarative grammar tree](0023-proposed-unified-grammar-tree.md) — **Proposed** (researched direction, not yet accepted)