fix: address verified gadfly P2 findings (9 real of 18)

Independently verified all 18 gadfly findings against the code (18-agent
fan-out). Fixed the 9 real ones; the other 9 were false-positive /
hallucinated / valid-tradeoff (no change).

High:
- F1 nil model: a Models resolver returning (ctx,nil,nil) flowed into the
  agent loop and nil-panicked. Now a clean error (Run never panics). +test.
- F9 compactor data-leak: renderTranscript sent tool-call args verbatim to
  the summarizer (a possibly-different provider/tier); secret-bearing tool
  args (mcp_call/email_send/http_*/webhook_*) are now redacted, with a doc
  note that result bodies still flow (summary needs them).

Medium/minor:
- F2 compactor error path returned the folded slice, not the original msgs
  (contradicting the documented non-fatal contract) -> return msgs.
- F3 RunStats.Status only ok/error; now timeout (DeadlineExceeded) /
  cancelled (Canceled) via statusFor. +test.
- F4 step-zip emitted empty-name "ghost" steps when results>calls; now pairs
  min(calls,results) only.
- F5 SetIteration was never called -> RunState.Iteration always 0; the step
  observer now updates it each loop.
- F6 matchPending fallback was LIFO; now FIFO (matches the per-key queue).
- F7 estimateTokens had no default arm (future Part kinds counted as 0);
  unknown parts now counted conservatively.
- F8 cloud_sync silently truncated >1MiB responses -> opaque JSON error; now
  a clear "response exceeded N bytes" via readCapped.
- F12 step observer captured the caller ctx; now the merged runCtx.
- F13 compaction onFire was nil (doc claimed it logged); now wired to
  audit LogEvent("compaction_fired").
- F11 (no pre-dispatch hook in majordomo) documented honestly as a known
  limitation; F18 UsageSink doc clarified cache tokens are subsets of input.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

model/sink.go

@@ -16,6 +16,11 @@ import (
 // UsageSink receives one record per successful Generate through a model parsed
 // by this package (ParseModelRequest / ParseModelForContext). Implement it to
 // meter or bill; the token detail mirrors majordomo's Response.Usage.
+//
+// IMPORTANT: cacheReadTokens and cacheWriteTokens are PORTIONS of inputTokens,
+// not independent additive values (they let a sink price cached vs fresh input
+// differently). A sink must NOT compute total = input+output+cacheRead+
+// cacheWrite — that double-counts the cached input.
 type UsageSink interface {
 	Record(ctx context.Context, model string, inputTokens, outputTokens, cacheReadTokens, cacheWriteTokens int)
 }