add ADR-0024: unified grammar tree execution plan (accepted)

Concrete specification for the direction in ADR-0023, landed
during the round-6 design pass. Resolves all four rounds of
open design questions: walker as single source of truth,
scannerless terminal vocabulary (~8 building blocks), typed
value slots with content validators, WalkContext for schema-
aware narrowing from day one, WalkOutcome multi-purpose
return, HintMode per-node, ranker as separate layer, static
+ dynamic sub-grammars, aliases as Word annotations,
IdentSource taxonomy, six-phase per-command migration with
chumsky and walker side-by-side during the transition.

Key shifts from ADR-0023's sketch:

- Lexer dissolves entirely. Walker operates on bytes directly.
  dsl/lexer.rs, dsl/keyword.rs go away in Phase F.
- Schema-aware parse from day one (not phased). Typed value
  slots reject mis-shaped input at parse time with localised
  wording. Completion narrows per column type.
- Sub-grammars: static (fn() -> Node) for composition;
  dynamic (fn(&WalkContext) -> Node) for schema-dependent
  expansion. No global named registry.
- Path-bearing commands: BarePath becomes a routine
  non-whitespace terminal. Paths with spaces require quoting
  via StringLit (UX simplification, aligns with standard CLI
  convention).
- 13-node taxonomy: Word, Punct, Ident, NumberLit, StringLit,
  BlobLit, Flag, BarePath, Choice, Seq, Optional, Repeated,
  DynamicSubgrammar.

Migration plan: Phase A (walker scaffolding + app-lifecycle
commands), Phase B (DDL without value literals), Phase C
(create table), Phase D (data commands with full schema
awareness -- the design's central claim landing), Phase E
(replay), Phase F (delete chumsky + lexer + legacy parser
modules, simplify catalog). Estimated ~4 sessions total.

Also: rename ADR-0023 from 0023-proposed-unified-grammar-tree.md
to 0023-unified-grammar-tree.md (git mv preserves history)
and update its status to reflect the direction-accepted-but-
superseded-for-execution-detail relationship with ADR-0024.
Index updated.

docs/adr/README.md

@@ -28,4 +28,5 @@ 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)
+- [ADR-0023 — Unified declarative grammar tree](0023-unified-grammar-tree.md) — direction (superseded for execution detail by ADR-0024)
+- [ADR-0024 — Unified grammar tree: execution plan](0024-unified-grammar-tree-execution-plan.md) — **Accepted**, the executable spec