ADR-0024 Phase F (minimal): drop chumsky from the parse path

Delete the chumsky-side command_parser and its per-command
sub-parsers, error humanisation helpers, and keyword/punct/
ident/value-literal combinators. The unified-grammar walker
in `crate::dsl::walker` is now the sole parse path.

parse_tokens flow (post-Phase F):
1. lex(input) — still produces the Token stream that
   completion / highlighting / echo-line consumers depend on.
2. try_walker_route(source) — walker handles every entry
   keyword in REGISTRY (20 commands across 14 entry words).
3. unknown_command_error(source) — synthetic ParseError for
   inputs whose first identifier-shape token isn't a
   registered entry word. Wording mirrors the chumsky-side
   "expected one of `add`, `change`, …, found `<word>`"
   structural error the legacy top-level Choice produced.

Cargo.toml: chumsky dependency dropped (no remaining uses).
Cargo.lock regenerated; ~58 lines net reduction in the
dependency graph.

Scope intentionally deferred (separate follow-up):
- dsl/lexer.rs, dsl/keyword.rs, dsl/ident_slot.rs,
  dsl/usage.rs::REGISTRY: still consumed by completion.rs,
  input_render.rs, app.rs, theme.rs, db.rs, runtime.rs,
  friendly/keys.rs. Removing these requires migrating each
  consumer to the walker's per-byte-class output / grammar
  REGISTRY / IdentSource enum. Substantial blast radius;
  worth a dedicated session.
- parse.token.keyword.* catalog entries (40+): used by
  usage.rs and parse-error rendering for the unmigrated
  consumers above. Collapse follows after the consumer
  migration.

Tests:
- All existing parser tests (`dsl::parser::tests`) ported in
  place; they call `parse_command` which now flows through
  the walker. 844 passed, 0 failed, 1 ignored — same count
  as Phase E (no test additions, no regressions).
- cargo clippy --all-targets -- -D warnings clean.
- cargo build (release-like dev profile) succeeds.

Cargo.toml

@@ -11,7 +11,6 @@ publish = false
 [dependencies]
 anyhow = "1.0.102"
 base64 = "0.22.1"
-chumsky = "0.13.0"
 crossterm = { version = "0.29.0", features = ["event-stream"] }
 csv = "1.4.0"
 directories = "6.0.0"