broker + repl + safety: GBNF grammar-sampling passthrough (closes #88)

llama.cpp constrains the sampler to ONLY emit tokens matching a
GBNF grammar. For small models this kills format drift at the
token level — `CMD: <cmd>` is enforced by the sampler rather than
hoped for via prompt discipline.

Probe finding (this commit's pre-implementation): cloud (Anthropic
via Bedrock) silently IGNORES the `grammar` field — returns normally
via standard sampling. Default passthrough is safe for all routes;
no per-model opt-in/opt-out needed in v1.

Changes:

- broker.lua build_request: `if opts.grammar then req.grammar =
  opts.grammar end`. Misformed grammar surfaces at request time
  via the existing transport-error path.

- repl.lua ask_ai: `grammar_override = config.routing.grammars
  [req_class]` (same gating shape as #86's system_prompts override).
  Passed via opts.grammar in the call_broker invocation.

- safety.lua is_destructive threads cfg.safety.probe_grammar through
  opts.grammar so llm_probe constrains the YES/NO output. Skips
  the regex-match dance entirely when the model can't drift.
  Caller-provided opts.grammar takes precedence over cfg.

- config.lua gains two commented examples:
  * routing.grammars per class
  * safety.probe_grammar for the destructive probe

6 unit cases verified (stubbed curl.post_sse / broker.chat):
  - default: no grammar in body
  - opts.grammar -> body contains grammar JSON-encoded
  - safety probe_grammar reaches llm_probe via opts
  - no probe_grammar configured -> opts.grammar nil
  - caller opts.grammar takes precedence over cfg.safety.probe_grammar

E2E against live local broker:
  - `routing.grammars.default = "root ::= \\"ACK\\""` configured;
    prompted "tell me a long story about a fox" -> model output
    EXACTLY "ACK" (sampler forced; would normally produce paragraphs).
    Grammar passthrough end-to-end confirmed.

Regression: test_safety 87/87, test_router_model 31/31, repl loads.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

broker.lua

@@ -61,6 +61,14 @@ local function build_request(model_cfg, messages, stream, opts)
     if stream and opts.include_usage ~= false then
         req.stream_options = { include_usage = true }
     end
+    -- #88: GBNF grammar passthrough. llama.cpp constrains the sampler
+    -- to only emit tokens matching the grammar — eliminates format
+    -- drift on small models. Probed cloud (Anthropic via Bedrock)
+    -- silently ignores the field, so default passthrough is safe;
+    -- no per-model opt-out needed in v1. Misformed grammar produces
+    -- a broker error at request time (visible via the usual transport
+    -- error path).
+    if opts.grammar then req.grammar = opts.grammar end
     return url, json.encode(req), build_headers(model_cfg),
            (model_cfg.timeout_ms or 60000)
 end