feat: add protocol source-of-truth schemas (TIER-1) (#52)

Introduce a new self-contained src/protocol/ directory that mirrors the
private factory-mono-alpha protocol schemas/enums exactly (zod-only, no
internal deps). This is the first step toward making droid-sdk-typescript
the single source of truth for the wire protocol, eliminating manual drift.

Covers TIER-1 foundation only: leaf enums, the JSON-RPC envelope family,
HostId, SessionTag, TokenUsage, loop schemas, and the message content-block
stack. Not yet wired into the public API (src/index.ts); existing files are
untouched. Adds tests/protocol-dir.test.ts round-trip coverage.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

Author: varin-nair-factory

src/protocol/constants.ts

@@ -0,0 +1,27 @@
+// Source-of-truth mirror of factory-mono-alpha protocol constants.
+// Faithful copy of:
+//   packages/common/src/droid/schemas/constants.ts
+//   packages/common/src/droid/constants.ts (LOOP_INTERVAL_POLICY)
+// Keep shapes/values identical to the private monorepo to avoid protocol drift.
+
+/**
+ * Legacy envelope field for old JSON-RPC peers - `factoryApiVersion` must be set to this value.
+ *
+ * @deprecated Do not change this value; use `factoryProtocolVersion` for runtime compatibility.
+ */
+export const LEGACY_FACTORY_API_VERSION = '1.0.0' as const;
+
+export const FACTORY_PROTOCOL_VERSION = '1.62.0' as const;
+
+export const JSONRPC_VERSION = '2.0' as const;
+
+const seconds = (value: number) => value * 1_000;
+const minutes = (value: number) => seconds(value * 60);
+const hours = (value: number) => minutes(value * 60);
+
+export const LOOP_INTERVAL_POLICY = {
+  minMs: seconds(5),
+  maxMs: hours(24),
+  displayRange: '5s–24h',
+  examples: '5s, 5m, 30m, 2h',
+} as const;