fix: OpenRouter 402 — send max_tokens cost cap; make session window configurable

User hit 'API error 402: ... You requested up to 131072 tokens, but can
only afford 4511' on the assist alias: no provider ever sent max_tokens,
so OpenRouter's credit pre-check billed the routed model's full
worst-case output; user also asked to bound session history to the last
5 requests/responses.

Architect decisions:
- AI_MAX_TOKENS (num, default 2048): sent as max_tokens on OpenRouter
  and generationConfig.maxOutputTokens on Gemini — a real per-request
  cost ceiling. llamacpp unchanged (local/free, no pre-check).
- AI_SESSION_TURNS (num, default 40 kept back-compat; messages, 2 per
  exchange — 10 = last 5 conversations): resolved lazily in session_push
  because config loads after the hardcoded line-25 default.
- Both registered in the bin/pos-ai POS_CONFIG @General section, so they
  appear in 'pos config ai' with num: validation.

Reviewer hardening (CHANGES_REQUIRED -> fixed): unguarded env input could
reach jq tonumber (0/-5/010/abc all savable via config-ui's ^-?[0-9]+$)
and abort the CLI; both providers and session_push now guard with
^[1-9][0-9]*$ and fall back to the default.

Verified: fake-curl shim smoke (16 provider-body + 12 session-window
checks incl. the 010-regression proof), make gen idempotent, make check
OK, make lint 0 FAIL/0 WARN, make test 17 files / 299 checks / 0 fail
(~49s), bash -n clean, git diff --check clean. Reviewer ACCEPT (twice).

Tester regression round (permanent provider-body + session-pruning
coverage) intentionally not run this cycle — user's call; remains a
documented follow-up.

lib/ai-providers/gemini.sh

@@ -14,8 +14,11 @@ provider_default_model() { printf 'gemini-2.5-flash'; }
 provider_generate() {
     local model="$1" messages="$2" system="${3:-}" body resp code body_out errmsg
     # Convert OpenAI messages format to Gemini contents format
-    body="$(printf '%s' "$messages" | jq -c '{
-        contents: [.messages[]? | {role: (.role | gsub("assistant";"model")), parts: [{text: .content}]}]
+    local mt="${AI_MAX_TOKENS:-2048}"
+    [[ "$mt" =~ ^[1-9][0-9]*$ ]] || mt=2048
+    body="$(printf '%s' "$messages" | jq -c --arg mt "$mt" '{
+        contents: [.messages[]? | {role: (.role | gsub("assistant";"model")), parts: [{text: .content}]}],
+        generationConfig: {maxOutputTokens: ($mt|tonumber)}
     }')"
     if [ -n "$system" ]; then
         body="$(printf '%s' "$body" | jq -c --arg s "$system" \

lib/ai-providers/openrouter.sh

@@ -19,8 +19,10 @@ provider_generate() {
     else
         body="$(printf '%s' "$messages" | jq -c '.messages')"
     fi
-    body="$(printf '%s' "$body" | jq -nc --arg m "$model" --argjson msgs "$body" \
-        '{model:$m, messages:$msgs}')"
+    local mt="${AI_MAX_TOKENS:-2048}"
+    [[ "$mt" =~ ^[1-9][0-9]*$ ]] || mt=2048
+    body="$(printf '%s' "$body" | jq -nc --arg m "$model" --argjson msgs "$body" --arg mt "$mt" \
+        '{model:$m, messages:$msgs, max_tokens:($mt|tonumber)}')"
     resp="$(curl -sS -m 60 -X POST "https://openrouter.ai/api/v1/chat/completions" \
         -H "Authorization: Bearer ${AI_API_KEY}" \
         -H "Content-Type: application/json" \