context + repl + config: per-class system_prompt override (closes #86)

Small local models follow precise structured instructions better than
natural language. Per-routing-class system_prompt override gives them
tighter instructions for THAT request while preserving ambient context.

Changes:

- Context:to_messages(opts) — opts.system_prompt_override REPLACES
  the base system_prompt for THIS render only (state unchanged).
  Dynamic blocks ([background], [project], [earlier summary], NORRIS
  suffix) still compose on top. opts is optional; nil-safe for old
  callers.

- repl.lua ask_ai — captures req_class from router.classify_model
  (already returned by Phase 5; previously discarded after the
  status line). Looks up config.routing.system_prompts[req_class];
  passes as opts.system_prompt_override to ctx:to_messages each
  iteration of the tool-sub-loop.

- Gating: override fires only when routing.auto is on (no class ->
  no override). If system_prompts[class] absent for a class, fall
  through to the default system_prompt (no surprise).

- Norris unaffected: safety.norris_step builds its own messages
  array; doesn't go through this path.

- config.lua gains a commented-out example showing routing.system_
  prompts with the code/default examples from the FR body.

Smoke verified:
  - 12-case context.lua unit test: opts nil/absent/present, override
    replaces base, dynamic blocks still compose, state unchanged
    after call, Norris-mode coexistence (suffix still present;
    background still suppressed).
  - E2E against cloud broker with routing.system_prompts.code set:
    triple-backtick prompt -> code class -> override fires; model
    emits terse code-only output. Non-code prompt -> default class
    -> no override -> normal verbose-ish reply.

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

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

config.lua

@@ -240,6 +240,26 @@ return {
     --     -- Toggle at runtime with :fallback on / :fallback off.
     --     fallback       = false,           -- default off (cost-safety)
     --     fallback_model = "cloud",
+    --
+    --     -- Issue #86: per-class system_prompt override. When the
+    --     -- classified request falls into a class with an entry here,
+    --     -- the BASE system_prompt is REPLACED for that one request
+    --     -- (dynamic blocks — [background], [project], [earlier
+    --     -- summary], NORRIS suffix — still compose on top). Mostly
+    --     -- useful for tightening small local models' instruction
+    --     -- adherence. Default {} (no override).
+    --     system_prompts = {
+    --         code = [[You are a code assistant. Rules:
+    -- 1. Output ONLY the requested code or command.
+    -- 2. No prose explanation unless explicitly asked.
+    -- 3. Wrap shell commands in CMD: prefix.
+    -- 4. Max response: 200 tokens.]],
+    --         default = [[You are a shell assistant.
+    -- Output shell commands as: CMD: <command>
+    -- Output answers as single short sentences.
+    -- Do not ask clarifying questions.]],
+    --         -- reasoning routes to cloud; no override usually needed
+    --     },
     -- },
 
     -- ── Phase 5 context summarization on sliding-window eviction.