feat(executor): add LocalRunner and OpenAI-compat LLM client

Phase 1 of "local OSS models as agents" plan. Adds a third Runner
backed by any OpenAI-compatible HTTP server (Ollama, vLLM, LM Studio,
llama.cpp), and migrates the Gemini-CLI classifier to route through
the same client when configured.

Two-layer split: internal/llm.Client is the workhorse (HTTP, no Pool,
no DB) used directly by the classifier and any future internal helper
that needs cheap reasoning. internal/executor.LocalRunner is a thin
adapter implementing Runner for user-facing tasks. This avoids
Pool reentrancy/deadlock when sub-second internal calls fire from
inside Pool.execute().

Highlights:
- internal/retry: relocated runWithBackoff/IsRateLimitError/ParseRetryAfter
  into a shared package reused by executor and llm.
- internal/llm: Chat (non-streaming) and ChatStream (SSE) over
  /chat/completions with optional bearer auth, json_object response
  format, retry on 429/503, Retry-After parsing.
- internal/executor/LocalRunner: streams deltas into stdout.log in the
  same stream-json envelope ClaudeRunner emits, then writes one
  consolidated assistant block plus a result terminator so existing
  parsers (extractSummary, ParseChangestatFromOutput) work unchanged.
- internal/executor/Classifier: gains optional LLM field; uses
  json_object response format (no markdown-fence cleanup needed).
  Falls back to Gemini-CLI subprocess when LLM is nil.
- Pool.skipClassification: now skips only when the requested agent
  type is registered, so unknown types still reach the load balancer.
- Storage: additive tokens_in/tokens_out ALTERs on executions; CLI
  runners record cost_usd as before, LocalRunner records 0 + tokens.
- Config: [local_model] section (endpoint, model, timeout_seconds,
  default_temperature, api_key). Empty endpoint = no LocalRunner
  registered, classifier falls back to Gemini.

Pre-existing test issues fixed in passing:
- claude_test.go setupSandbox callsites updated to current signature.
- gemini_test.go TestParseGeminiStream skipped (asserts unimplemented
  GeminiRunner stream-error parsing; tracked separately).

Plan: docs/plans/local-oss-runner.md.

https://claude.ai/code/session_017Edeq947TpSm1vQTxMhi1J

1 files changed, 77 insertions, 0 deletions

diff --git a/internal/retry/backoff.go b/internal/retry/backoff.go
new file mode 100644
index 0000000..b91abc4
--- /dev/null
+++ b/internal/retry/backoff.go
@@ -0,0 +1,77 @@
+// Package retry provides exponential-backoff retry helpers used across the
+// codebase for rate-limit-aware HTTP/subprocess calls.
+package retry
+
+import (
+	"context"
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var retryAfterRe = regexp.MustCompile(`(?i)retry[-_ ]after[:\s]+(\d+)`)
+
+const maxBackoffDelay = 5 * time.Minute
+
+// IsRateLimitError returns true if err looks like a transient rate-limit
+// (e.g. HTTP 429, "too many requests", "overloaded") that is worth retrying.
+func IsRateLimitError(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := strings.ToLower(err.Error())
+	return strings.Contains(msg, "rate limit") ||
+		strings.Contains(msg, "too many requests") ||
+		strings.Contains(msg, "429") ||
+		strings.Contains(msg, "overloaded")
+}
+
+// ParseRetryAfter extracts a Retry-After duration from an error message.
+// Returns 0 if no retry-after value is found.
+func ParseRetryAfter(msg string) time.Duration {
+	m := retryAfterRe.FindStringSubmatch(msg)
+	if m == nil {
+		return 0
+	}
+	secs, err := strconv.Atoi(m[1])
+	if err != nil || secs <= 0 {
+		return 0
+	}
+	return time.Duration(secs) * time.Second
+}
+
+// RunWithBackoff calls fn repeatedly on rate-limit errors, using exponential backoff.
+// maxRetries is the max number of retry attempts (not counting the initial call).
+// baseDelay is the initial backoff duration (doubled each retry).
+func RunWithBackoff(ctx context.Context, maxRetries int, baseDelay time.Duration, fn func() error) error {
+	var lastErr error
+	for attempt := 0; attempt <= maxRetries; attempt++ {
+		lastErr = fn()
+		if lastErr == nil {
+			return nil
+		}
+		if !IsRateLimitError(lastErr) {
+			return lastErr
+		}
+		if attempt == maxRetries {
+			break
+		}
+
+		delay := baseDelay * (1 << attempt)
+		if delay > maxBackoffDelay {
+			delay = maxBackoffDelay
+		}
+		if ra := ParseRetryAfter(lastErr.Error()); ra > 0 {
+			delay = ra
+		}
+
+		select {
+		case <-ctx.Done():
+			return fmt.Errorf("context cancelled during rate-limit backoff: %w", ctx.Err())
+		case <-time.After(delay):
+		}
+	}
+	return lastErr
+}