feat: allow compaction model override via config (#38753)

Merged via squash.

Prepared head SHA: a3d6d6c845
Co-authored-by: starbuck100 <25417736+starbuck100@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

docs/gateway/configuration-reference.md

@@ -1005,6 +1005,7 @@ Periodic heartbeat runs.
         identifierPolicy: "strict", // strict | off | custom
         identifierInstructions: "Preserve deployment IDs, ticket IDs, and host:port pairs exactly.", // used when identifierPolicy=custom
         postCompactionSections: ["Session Startup", "Red Lines"], // [] disables reinjection
+        model: "openrouter/anthropic/claude-sonnet-4-5", // optional compaction-only model override
         memoryFlush: {
           enabled: true,
           softThresholdTokens: 6000,
@@ -1021,6 +1022,7 @@ Periodic heartbeat runs.
 - `identifierPolicy`: `strict` (default), `off`, or `custom`. `strict` prepends built-in opaque identifier retention guidance during compaction summarization.
 - `identifierInstructions`: optional custom identifier-preservation text used when `identifierPolicy=custom`.
 - `postCompactionSections`: optional AGENTS.md H2/H3 section names to re-inject after compaction. Defaults to `["Session Startup", "Red Lines"]`; set `[]` to disable reinjection. When unset or explicitly set to that default pair, older `Every Session`/`Safety` headings are also accepted as a legacy fallback.
+- `model`: optional `provider/model-id` override for compaction summarization only. Use this when the main session should keep one model but compaction summaries should run on another; when unset, compaction uses the session's primary model.
 - `memoryFlush`: silent agentic turn before auto-compaction to store durable memories. Skipped when workspace is read-only.
 
 ### `agents.defaults.contextPruning`