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.

gateway/relay/descriptor.py

@@ -75,3 +75,44 @@ class CapabilityDescriptor:
         known = {f for f in cls.__dataclass_fields__}  # type: ignore[attr-defined]
         filtered = {k: v for k, v in raw.items() if k in known}
         return cls(**filtered)
+
+    @classmethod
+    def from_platform_entry(
+        cls,
+        entry,
+        *,
+        len_unit: str = "chars",
+        supports_draft_streaming: bool = False,
+        supports_edit: bool = True,
+        supports_threads: bool = False,
+        markdown_dialect: str = "plain",
+    ) -> "CapabilityDescriptor":
+        """Project a ``gateway.platform_registry.PlatformEntry`` into a descriptor.
+
+        Demonstrates the descriptor is a *subset/projection* of what
+        ``PlatformEntry`` already encodes, not a parallel concept: ``label``,
+        ``max_message_length``, ``emoji``, ``platform_hint``, ``pii_safe`` and
+        the platform name come straight off the entry. The runtime capability
+        bits that ``PlatformEntry`` does NOT encode (length unit, draft/edit/
+        thread/markdown behavior) are supplied by the caller — in production
+        the connector fills these from the live adapter's capability methods.
+
+        ``max_message_length`` of 0 on a ``PlatformEntry`` means "no limit";
+        we map that to the stream_consumer default of 4096 so the descriptor
+        always carries a concrete chunking bound.
+        """
+        max_len = getattr(entry, "max_message_length", 0) or 4096
+        return cls(
+            contract_version=CONTRACT_VERSION,
+            platform=entry.name,
+            label=entry.label,
+            max_message_length=max_len,
+            supports_draft_streaming=supports_draft_streaming,
+            supports_edit=supports_edit,
+            supports_threads=supports_threads,
+            markdown_dialect=markdown_dialect,
+            len_unit=len_unit,
+            emoji=getattr(entry, "emoji", "\U0001f50c"),
+            platform_hint=getattr(entry, "platform_hint", ""),
+            pii_safe=getattr(entry, "pii_safe", False),
+        )

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