Add the Agent Factories authoring surface

Agent Factories let a trusted extension declare a JavaScript closure that
orchestrates a fleet of subagents, and invoke it by name. The runtime half
shipped in copilot-agent-runtime#12953 and #13077; this is the SDK half.
The feature is dark behind the runtime's `agent_factories` flag plus its
billing gate, and every public type is marked `@experimental`.

RPC surface
- Bumps `@github/copilot` to ^1.0.75, the first published runtime whose
  schemas carry the full factory surface, and regenerates every language.
- Generated wire types for the factory session methods (`run`, `resume`,
  `getRun`, `listRuns`, `getRunDetail`, `getRunProgress`, `cancel`), the
  reverse `factory.execute` / `factory.abort` calls, run envelopes,
  registration metadata, and the observability DTOs.
- Fixes opaque-JSON codegen so a factory's args and result can be any JSON
  value rather than an object alone, which also lets a void factory
  complete without a result.

Authoring harness
- `defineFactory({ meta, run })` returns an opaque handle; an extension
  registers it through `joinSession({ factories: [...] })`. Only metadata
  crosses to the runtime — the closure stays in the extension process and
  is invoked over the reverse `factory.execute` RPC.
- The `run()` context: `args`, the extension's full `session`, a per-run
  `signal`, `runId`, plus `agent()` (one single-turn subagent, optionally
  schema-constrained), `step()` (journal-backed memoization), `parallel()`,
  `pipeline()`, and `phase()` / `log()` progress markers. `factory()`
  throws, because nesting is forbidden.
- `session.factory.*` friendly wrappers, including `resume(runId)`, which
  reuses the persisted name, arguments, journal, and accounting so a caller
  never re-sends them.
- Declared limits (`maxConcurrentSubagents`, `maxTotalSubagents`,
  `timeoutSeconds`, `maxAiCredits`) validated at the authoring boundary.
- Strict JSON validation shared by factory results and journaled `step()`
  producers, so a lossy value is rejected rather than silently mutating on
  a resumed replay.

`run()` and `resume()` resolve with the run envelope for every outcome —
completed, error, halted, or cancelled — and reject only when no run
exists (an unknown factory, a declined approval, an already-active
session). This keeps the signature stable for the planned background-only
execution mode, which changes a run's timing and status but not its shape.

Authoring documentation lives in `nodejs/docs/factories.md`, with a
pattern library in `nodejs/docs/factory-patterns.md`.

Author: MRayermannMSFT