Enable maximum-strict TypeScript across our packages

tsconfig.base.json adds noUncheckedIndexedAccess,
exactOptionalPropertyTypes, noImplicitOverride,
noFallthroughCasesInSwitch, noUnusedLocals, and noUnusedParameters on
top of strict. Vendored packages opt out of the new flags locally
(their tsconfigs are ours to regenerate; their source is not), keeping
upstream-sync friendliness.

Our code fixed accordingly: index accesses acknowledge undefined
(assembler flush cursors, lastTurnNumber); optional properties are
omitted instead of set-to-undefined (GenerateResult.usage,
ToolDefinition.strict, GenerateOptions.system/tools, error payloads
via an errorData helper); Session.onAppend is explicitly
`(…) => void | undefined`; tests and examples updated for unused
parameters and indexed access.

packages/session/src/index.ts

@@ -57,8 +57,8 @@ function renderTagged(tag: string, content: ContentBlock[], source: MessageSourc
  */
 export class Session {
   private log: SessionEvent[] = []
-  /** Set by the store so appends are observable; no-op when detached. */
-  onAppend?: (event: SessionEvent) => void
+  /** Set by the store so appends are observable; undefined when detached. */
+  onAppend: ((event: SessionEvent) => void) | undefined
 
   constructor(public readonly id: string, seed?: SessionEvent[]) {
     if (seed) this.log = [...seed]