feat(session): refuse session logs a build cannot faithfully read

Old runtimes meeting a newer session format now fail loud instead of
misreading: version refusal names the direction (newer: upgrade the
harness; older: no upgrade path) and points at the raw JSONL log, and an
event type outside the generated known vocabulary refuses resume unless
its envelope carries the new ignorable: true marker (default: required,
so a forgotten marker over-refuses instead of silently resuming a gutted
session). gen-persistence-catalog now also emits
KNOWN_SESSION_EVENT_TYPES; SQLite stores the marker in a dedicated
column (SCHEMA_VERSION 15). The versioning design (monotonic integer,
n->n+1 upgrader chain, migrate-on-continue) is recorded in the
session-log-version-mechanism Agent Note.

packages/host/apiproxy/src/api/sessions.schema.ts

@@ -45,6 +45,7 @@ export const sessionEventSchema = z.object({
   data: z.unknown(),
   sourceEventSeqs: z.array(z.number()).optional(),
   surfaceOp: z.unknown().optional(),
+  ignorable: z.literal(true).optional(),
 }) as unknown as z.ZodType<SessionEvent>
 
 /** SessionSummary row of session.list (`projections` reuses the history block's shape and schema). */