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/core/session/tests/session.spec.ts

@@ -1090,12 +1090,20 @@ describe('Session', () => {
       { ...base, time: '1' },
       { ...base, time: 0.5 },
       { type: base.type, seq: base.seq, time: base.time },
+      { ...base, ignorable: false },
+      { ...base, ignorable: 'yes' },
     ]
 
     for (const [index, event] of cases.entries()) {
       expect(() => Session.create(SessionId(`bad-envelope-${index}`), [event as SessionEvent]))
         .toThrow(/invalid event envelope/)
     }
+
+    // `ignorable: true` is the one accepted marker value (unknown-type skip contract).
+    const marked = Session.create(SessionId('ignorable-envelope'), [
+      { ...base, ignorable: true } as SessionEvent,
+    ])
+    expect(marked.events[0]?.ignorable).toBe(true)
   })
 })