test(agent): raise the eager tool schema ceiling to 3650

main is red. TestEagerToolSchemaTokenBudget fails at 3578 tokens against
a 3550 ceiling, and every open PR inherits it.

Bisected: edf660ab and 4ca4fa75 pass, cd0eb194 (#843, which added
view_image to the core set) fails at 3588. #867 later trimmed it to 3578.
#843's own CI was green, because the ceiling is only crossed once the
tool set is combined, so no per-PR check could have caught it.

Raised rather than paid down. The overrun is 28 tokens, and view_image
costs 82, the second cheapest tool in the set against exec_command's 909
and request_permissions' 397. It is simply the one that crossed the line.
The alternative was trimming a description to reclaim 28 tokens,
degrading a tool's usability to satisfy a line drawn against a 2026-07
measurement that predates two tools. The ratchet worked exactly as
intended: it caught the creep and forced a decision instead of a drift.

Also corrects the failure message. It advised "defer a tool", but this
test pins DeferThreshold at 0 so deferral is inactive and marking a tool
deferred leaves it exposed here, changing nothing. Deferring is still
worth doing for real sessions; it just cannot move this number. The
levers that can are a smaller schema, one fewer core tool, or a
deliberate raise, and the message now says so.

internal/agent/prompt_budget_test.go

@@ -20,7 +20,20 @@ import (
 // without a session id. Higher-risk tools that do not opt into auto remain excluded.
 const (
 	maxBaseSystemPromptTokens = 3500
-	maxEagerToolSchemaTokens  = 3550
+	// Raised deliberately from 3550 on 2026-08-08. view_image (#843) took the set
+	// to 3588, and #867 trimmed it back to 3578: still 28 tokens over.
+	//
+	// The overrun is a rounding error rather than a regression. view_image costs
+	// 82 tokens, the second cheapest tool in the set, against exec_command's 909
+	// and request_permissions' 397; it is simply the one that crossed the line.
+	// Raised rather than paid down because the alternative was trimming a
+	// description to claw back 28 tokens, degrading a tool's usability to satisfy
+	// a line drawn against a 2026-07 measurement that predates two tools.
+	//
+	// The ratchet did its job: it caught the creep and forced a decision instead
+	// of a drift. That is the point of it, so keep raising it deliberately rather
+	// than reflexively.
+	maxEagerToolSchemaTokens = 3650
 )
 
 func TestSystemPromptTokenBudget(t *testing.T) {
@@ -46,6 +59,11 @@ func TestEagerToolSchemaTokenBudget(t *testing.T) {
 	got := estimateToolDefTokens(exposed)
 	t.Logf("eager core tool schemas: %d tokens across %d tools", got, len(exposed))
 	if got > maxEagerToolSchemaTokens {
-		t.Fatalf("eager tool schemas are %d tokens, over the %d ceiling — defer a tool or raise the ceiling deliberately", got, maxEagerToolSchemaTokens)
+		// NOT "defer a tool": this test pins DeferThreshold at 0, so marking a
+		// tool deferred leaves it exposed here and changes nothing. Deferral is
+		// still worth doing for real sessions, it just cannot move this number.
+		// The levers that do are a smaller schema, one fewer core tool, or a
+		// deliberate raise.
+		t.Fatalf("eager tool schemas are %d tokens, over the %d ceiling — trim a schema, drop a core tool, or raise the ceiling deliberately (deferring will NOT help: this test disables deferral)", got, maxEagerToolSchemaTokens)
 	}
 }