context + repl + config: route-aware context compression (closes #87)

Small local models effectively use a fraction of their advertised
context window. Per-request compression for routes that hit a
local-compress-flagged model preset: keeps only the last N turns
and tail-truncates oversized content. Cloud routes get the full
context unchanged.

Changes:

- context.lua _compress_turns(turns, keep, max_chars): returns a
  new list (self.turns NEVER mutated) with the last `keep` turns
  preserved + content tail-truncated to `max_chars`. Defensive:
  drops tool turns at the slice head (orphaned without their
  assistant-with-tool_calls anchor — strict chat templates would
  reject them; same gotcha PHASE0 §6 warned about for user/user).

- Context:to_messages(opts) — opts.compress = { keep_turns,
  max_turn_chars } swaps the turn iterable for the compressed
  view. Affects BOTH the use_tool_role=true path and the
  use_tool_role=false fallback (PHASE2.md Q18 strict-template
  workaround). Persistence + display via :history see the full
  uncompressed ctx.turns.

- repl.lua ask_ai: when req_cfg (the routed model's cfg) has
  `local_compress = true`, build compress_opts from
  config.context.compress (defaults keep_turns=2, max_turn_chars=800).
  Pass through ctx:to_messages alongside the existing
  system_prompt_override (#86) — orthogonal opts that compose.

- Norris unaffected: safety.norris_step builds its own messages
  array; the planner needs full history per PHASE3 design.

- config.lua gains a header comment explaining the per-model opt-in
  + the context.compress defaults block + the documented tool-turn
  truncation trade-off.

13 unit cases verified:
  - no opts -> full turn list (no regression)
  - keep_turns=2 -> exactly last 2 emitted
  - long content tail-truncated to max_chars
  - self.turns unchanged after render
  - orphan tool-turn at slice head dropped (no chat-template violation)
  - tool turn included WITH its assistant anchor when keep_turns >= 3

E2E against live local broker:
  - models.fast.local_compress = true; keep_turns=1; max=200
  - 4-turn session: each broker call sees ONLY the current turn
    (verified by short coherent CMD replies despite no cross-turn
    memory available to the model). FR-promised small-model
    friendliness in action; conversation continuity is the
    documented trade-off.

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

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

repl.lua

@@ -1007,6 +1007,20 @@ function M.run(config)
                              and config.routing.grammars
                              and req_class
                              and config.routing.grammars[req_class]
+        -- #87: route-aware context compression. When the routed model
+        -- preset has `local_compress = true`, ctx:to_messages keeps only
+        -- the last N turns and tail-truncates oversized content for
+        -- THIS request. Cloud routes (model_cfg.local_compress nil/false)
+        -- get the full context unchanged. Defaults from cfg.context.compress;
+        -- per-model opt-in keeps the design surface predictable.
+        local compress_opts
+        if req_cfg and req_cfg.local_compress then
+            local cc = (config.context and config.context.compress) or {}
+            compress_opts = {
+                keep_turns     = cc.keep_turns     or 2,
+                max_turn_chars = cc.max_turn_chars or 800,
+            }
+        end
 
         local depth = 0
         local final_resp = ""
@@ -1017,7 +1031,10 @@ function M.run(config)
             local tool_calls_seen = {}
             local redact_mode     = secrets_mode_for(req_cfg)
             local scrubbed_msgs   = scrub_messages(
-                ctx:to_messages({ system_prompt_override = sys_override }),
+                ctx:to_messages({
+                    system_prompt_override = sys_override,
+                    compress               = compress_opts,
+                }),
                 redact_mode)
             -- Streaming rehydrator wraps the on_delta so the user sees real
             -- values; text_parts accumulates the REHYDRATED chunks so