fix(agents): make post-compaction guard config valid + observation trim-resilient

Two correctness fixes from code review.

1. Zod schema (src/config/zod-schema.agent-runtime.ts) was strict and
   rejected tools.loopDetection.postCompactionGuard.* keys at validation
   time, making the guard's documented configurability inaccessible at
   gateway startup. Adds ToolLoopPostCompactionGuardSchema with both
   optional fields and wires it into ToolLoopDetectionSchema.

2. The runner observation cursor in pi-embedded-runner/run.ts used
   absolute indices into state.toolCallHistory, but that array is
   trimmed at historySize (default 30). Once the buffer was full, new
   records shifted out from under the cursor and the guard silently
   missed every loop in long-running sessions. Replaces the index
   cursor with a monotonic toolOutcomeSeq on SessionState that
   recordToolCallOutcome bumps on each observable push (unmatched
   branch only, mirroring the prior cursor's effective semantics).
   The runner now reads the most recent (currentSeq - lastSeq) entries
   from the tail of toolCallHistory, which is trim-resilient.

Adds zod parse tests for the new config keys (valid, empty, unknown
key, non-positive, non-integer) and a runner regression test that
seeds toolCallHistory at the trim cap before triggering a
post-compaction loop, asserting the abort still fires.

Refs #77474

docs/.generated/config-baseline.sha256

@@ -1,4 +1,4 @@
-7526a2ec39c5bbe905f8347f900d812f28dd65caaf3f0172d1e9692fb186dcd3  config-baseline.json
-baa4a12a48a03755cab302629a1f6b65b2e826c091a987d68de5989d39e8ed35  config-baseline.core.json
+a963f7242f75fbc137a7891b8056c199b42eeb8f4887aeaaa079bff6394feb89  config-baseline.json
+51f525cfffa40659d6273e56705e91f3cb1255b217dadab59c32f77e0f770b37  config-baseline.core.json
 cd7c0c7fb1435bc7e59099e9ac334462d5ad444016e9ab4512aae63a238f78dc  config-baseline.channel.json
 9832b30a696930a3da7efccf38073137571e1b66cae84e54d747b733fdafcc54  config-baseline.plugin.json