feat(relay): derive descriptor from PlatformEntry

CapabilityDescriptor.from_platform_entry() projects an existing PlatformEntry
(label, max_message_length, emoji, platform_hint, pii_safe, name) into a
descriptor, proving the descriptor is a projection of existing config rather
than a parallel concept. Runtime-only capabilities (len_unit, draft/edit/
thread/markdown) are caller-supplied. max_message_length==0 ('no limit') maps
to the stream_consumer 4096 default.

Phase 0 complete. Task 0.3 of the gateway-relay plan.

tests/gateway/relay/test_descriptor_from_entry.py

@@ -0,0 +1,64 @@
+"""Descriptor <- PlatformEntry projection (relay Phase 0, Task 0.3).
+
+Proves the CapabilityDescriptor is a projection of the existing PlatformEntry,
+not a parallel concept: the entry's label/limit/emoji/hint/pii fields carry
+straight through.
+"""
+
+from gateway.platform_registry import PlatformEntry
+from gateway.relay.descriptor import CONTRACT_VERSION, CapabilityDescriptor
+
+
+def _entry(**overrides) -> PlatformEntry:
+    base = dict(
+        name="telegram",
+        label="Telegram",
+        adapter_factory=lambda cfg: None,
+        check_fn=lambda: True,
+        max_message_length=4096,
+        pii_safe=False,
+        emoji="\u2708\ufe0f",
+        platform_hint="You are on Telegram.",
+    )
+    base.update(overrides)
+    return PlatformEntry(**base)
+
+
+def test_projection_carries_platform_entry_fields():
+    d = CapabilityDescriptor.from_platform_entry(_entry(), len_unit="utf16")
+    assert d.contract_version == CONTRACT_VERSION
+    assert d.platform == "telegram"
+    assert d.label == "Telegram"
+    assert d.max_message_length == 4096
+    assert d.emoji == "\u2708\ufe0f"
+    assert d.platform_hint == "You are on Telegram."
+    assert d.pii_safe is False
+    assert d.len_unit == "utf16"
+
+
+def test_zero_max_length_maps_to_4096_default():
+    """PlatformEntry.max_message_length == 0 means 'no limit'; the descriptor
+    carries a concrete bound matching the stream_consumer default."""
+    d = CapabilityDescriptor.from_platform_entry(_entry(max_message_length=0))
+    assert d.max_message_length == 4096
+
+
+def test_runtime_capabilities_supplied_by_caller():
+    """PlatformEntry doesn't encode draft/edit/thread/markdown behavior — those
+    come from the caller (the connector, reading the live adapter)."""
+    d = CapabilityDescriptor.from_platform_entry(
+        _entry(),
+        supports_draft_streaming=True,
+        supports_edit=False,
+        supports_threads=True,
+        markdown_dialect="discord",
+    )
+    assert d.supports_draft_streaming is True
+    assert d.supports_edit is False
+    assert d.supports_threads is True
+    assert d.markdown_dialect == "discord"
+
+
+def test_projection_roundtrips_through_json():
+    d = CapabilityDescriptor.from_platform_entry(_entry(), len_unit="utf16")
+    assert CapabilityDescriptor.from_json(d.to_json()) == d