docs(server-beta): state what the summary input budget does not cover

Greptile flagged that the budget compares event-block bytes while
buildServerGenerationPrompt goes on to add the request wrapper, the
project/session/job tags, the instructions and the observation schema.
Correct: built with zero events, that envelope measures 1,244 bytes here.

Left out of the budget rather than reserved from it. Unlike the payload,
the envelope is a fixed cost -- it scales with neither event count nor
payload size -- so at the 600 KB default it is 0.2%, and the only budgets
it could decide are ones already too small to produce a useful summary
(~1 KB buys a single truncated tool call).

Documented instead of enforced because every additional thing this
function knows about the prompt is another thing that can drift out of
sync with the builder, which is what produced the bug this PR fixes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: cherry-pick-ops

src/server/generation/ProviderObservationGenerator.ts

@@ -73,6 +73,13 @@ export interface ProviderObservationGeneratorOptions {
 // close carries the outcome and what was left pending. Dropping the middle
 // yields a partial summary, which is strictly better than the current outcome
 // for these sessions — an unrecoverable failure and no summary at all.
+// Bounds the SESSION INPUT, not the finished prompt: buildServerGenerationPrompt
+// wraps the selected event blocks in request/project/job tags, the instructions and
+// the observation schema. That envelope is a fixed cost — 1,244 bytes with the mode
+// active here, independent of event count and payload size — so it is left out of
+// the budget rather than reserved from it. At the default that is 0.2%; only a
+// budget small enough to be unusable anyway (~1 KB buys one truncated tool call)
+// would be decided by it.
 const SUMMARY_INPUT_BUDGET_BYTES = Number.parseInt(
   process.env.CLAUDE_MEM_SUMMARY_INPUT_BUDGET_BYTES ?? '',
   10,