script: add Grammar() discovery — front ends query the grammar, never embed it

[vinodhalaharvi-claude]

What

Grammar() is a single discovery entry point: a front end asks "what can be done?", pipes the result into translation, and forwards the output — never inspecting the contents, never naming a registry, never hardcoding a verb list. When AgentScript adds verbs or restructures, the front end reflects it automatically, because it fetches the grammar instead of embedding knowledge of it.

AgentScript is the source of truth; the front end is a dumb pipe.

Why

Even after the complete-registry work, a front end calling CompleteRegistry() is still naming a specific registry — it knows too much, and breaks if that name/structure changes. Discovery closes that leak: loom calls Grammar() and pipes it through. agentscript can evolve the grammar freely; loom never changes.

New: pkg/script/grammar.go

• GrammarInfo { Registry, Verbs, Operators, Backends } — self-describing discovery result. Registry goes straight to Translate/Compile (callers don't inspect it); Verbs/Operators/Backends are advisory metadata for help/docs/CLI surfaces.
• Grammar() GrammarInfo — THE discovery entry, backed by CompleteRegistry. Exposes all 112 verbs, the >=> / <*> operators, and the memory/temporal backends.

New: pkg/script/submit.go

• TranslateGrammar(ctx, complete, GrammarInfo, prose) — discovery-driven translate: takes GrammarInfo, not a registry, so a front end pipes Grammar() straight in without ever touching or naming a registry.
• CompileGrammar(ctx, GrammarInfo, src) — matching compile entry.

The decoupling, concretely

loom's flow becomes:

g := script.Grammar()                          // ask: what can be done?
src, _ := script.TranslateGrammar(ctx, llm, g, prose)  // pipe through — never inspect g
plan, _ := script.CompileGrammar(ctx, g, src)

loom names no registry, reads no verb list, and auto-reflects any future grammar change.

Tests (grammar_test.go)

• Grammar exposes the full sorted vocabulary + representative families (echo, hf_summarize, mcp_agent, perplexity)
• describes >=> / <*> and both backends
• the discovery-driven Translate+Compile path works end-to-end without naming a registry
• a historical verb emitted under temporal surfaces NotImplementedOnBackendError (known, not unknown) — proving discovery composes with the per-backend availability contract

Step	Result
go vet -structtag=false ./...	✅
gofmt / staticcheck	✅
go test -race ./...	✅
go build ./...	✅

Purely additive; existing API unchanged.

Next

loom PR — call Grammar() + TranslateGrammar/CompileGrammar instead of naming DefaultRegistry. loom stops knowing the grammar entirely; gains the full 112-verb vocabulary automatically. (Execution of non-echo verbs on temporal still reports 'not implemented yet' until step 2 wires memory execution / verbs get temporal activities — honest, forward-compatible.)