feat(session): metadata seam + JSON-serializability invariant

Adds the durable-session metadata seam and enforces the log's
JSON-serializability invariant at the source:

- SessionHeader / SessionSummary / SessionMeta and CreateSessionOptions in
  dsh-session; Session gains a readonly `header`; SessionStore.create takes
  `(id?, options?: { seed?; meta? })` (validated absolute cwd, parentSession
  lineage). The injection TurnTrigger variant is added for the idle-inject
  one-shot turn that a later change introduces.
- isJsonValue (new json.ts): a value round-trips through JSON losslessly —
  rejects BigInt, function, symbol, undefined, non-finite numbers, sparse
  arrays, circular refs, and exotic objects (Map/Set/Date/class instances).
- Session.append throws on non-JSON-serializable data, and the Session
  constructor validates every seed event (isJsonValue + contiguous seq from
  0), so a replay/fork seed can never build a live log no backend can
  persist — the source-level guarantee a durable backend relies on.

Migrates the ~3 internal positional-seed `create(id, seed)` call sites to
`{ seed }`, and adapts the invariants tests forced by the new guard (the
bad-seq seed is now caught by the constructor; the cyclic deep-freeze test
drives via session/event since append rejects cyclic data; a direct
session/event drives the invariants seq-monotonicity check). Docs kept
backend-agnostic (the persistence packages arrive in a later PR).

packages/agent-loop/tests/loop.spec.ts

@@ -425,7 +425,7 @@ describe('agent loop', () => {
     send(agent, 'run')
     await waitForIdle(ctx, agent)
 
-    const replayed = ctx.sessions.create('replayed', [...agent.session.events])
+    const replayed = ctx.sessions.create('replayed', { seed: [...agent.session.events] })
     expect(replayed.deriveMessages()).toEqual(agent.session.deriveMessages())
     // event-by-event identity of types
     expect(replayed.events.map(e => e.type)).toEqual(