feat(compaction): make post-compaction context sections configurable (#34556)

Merged via squash.

Prepared head SHA: 491bb28544
Co-authored-by: efe-arv <259833796+efe-arv@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

docs/gateway/configuration-reference.md

@@ -1003,6 +1003,7 @@ Periodic heartbeat runs.
         reserveTokensFloor: 24000,
         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
         memoryFlush: {
           enabled: true,
           softThresholdTokens: 6000,
@@ -1018,6 +1019,7 @@ Periodic heartbeat runs.
 - `mode`: `default` or `safeguard` (chunked summarization for long histories). See [Compaction](/concepts/compaction).
 - `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.
 - `memoryFlush`: silent agentic turn before auto-compaction to store durable memories. Skipped when workspace is read-only.
 
 ### `agents.defaults.contextPruning`