fix(kg): port source_drawer_id provenance with schema migration (#1314)

[quangdang46]

Summary

Follow-up to #5. Ports the schema-changing part of upstream e4e25ed (milla-jovovich/mempalace#1314, RFC 002 §5.5) that the previous PR explicitly deferred because it requires an ALTER TABLE on the triples table. With this PR, the gap entry for source_drawer_id in port.txt is now resolved.

What changed

• Schema: triples now declares source_drawer_id TEXT and adapter_name TEXT in the canonical CREATE TABLE. For palaces created before these columns existed, a new KnowledgeGraph::migrate_schema introspects PRAGMA table_info(triples) and only issues ALTER TABLE ADD COLUMN when the column is missing — mirroring the upstream _migrate_schema no-op-on-new-installs contract. Fresh palaces pay no cost; legacy palaces are migrated in-place on open().
• API: KnowledgeGraph::add_triple gains two trailing Option<&str> kwargs (source_drawer_id, adapter_name), defaulting to None so all existing call sites stay source-compatible. Triple and EntityQueryResult expose source_drawer_id; Triple also exposes adapter_name. New fields use #[serde(default, skip_serializing_if = "Option::is_none")] so existing JSON consumers stay backward compatible.
• MCP: tool_kg_add accepts a new source_drawer_id input and forwards it to the KG layer; the tool input schema advertises the property with a description that references RFC 002 §5.5. adapter_name is intentionally not exposed at the MCP boundary — matching upstream, where only adapters set it.
• Tests: 4 new regression tests (3 unit + 1 MCP integration):
  • knowledge_graph::tests::test_add_triple_persists_source_drawer_and_adapter — round-trip persistence of both new columns.
  • knowledge_graph::tests::test_query_entity_exposes_source_drawer_id — query_entity and timeline surface the drawer id.
  • knowledge_graph::tests::test_migrate_schema_adds_missing_provenance_columns — creates a legacy triples table without the new columns and verifies KnowledgeGraph::open migrates it and that subsequent writes use the migrated columns.
  • mcp_server::tests::test_kg_add_forwards_source_drawer_id — mirrors upstream test_kg_add_forwards_source_provenance; reads the raw row to confirm the drawer id reaches SQLite via the MCP boundary.
• port.txt: removes the source_drawer_id entry from "Remaining gaps" and appends a follow-up sync entry documenting what was ported.

Review & Testing Checklist for Human

• Verify the in-place migration on an existing palace. Open a palace created with this repo before this PR (or with the upstream Python version) and confirm that PRAGMA table_info(triples) shows the new source_drawer_id and adapter_name columns after KnowledgeGraph::open runs. The migration is idempotent — running open() twice is a no-op on the second call.
• Confirm backward compatibility of the JSON shape. Existing consumers of Triple / EntityQueryResult JSON should still see no breaking change because the new fields are Option<String> with skip_serializing_if = "Option::is_none". Spot-check mempalace_kg_query output for facts written without the new fields.
• Check the MCP mempalace_kg_add schema — source_drawer_id should be visible as an optional string property in tools/list output.

Test plan to verify end-to-end:

1. cargo build --workspace
2. cargo test --workspace — expect 408 passed; 0 failed (4 new regression tests in mempalace-core).
3. cargo clippy --workspace --all-targets -- -D warnings — expect clean.
4. cargo fmt --all -- --check — expect clean.
5. Smoke-test the MCP boundary:
   • Call mempalace_kg_add with source_drawer_id="drawer_xyz", then query the triples row directly via SQLite and verify the column was persisted.
   • Query the same fact via mempalace_kg_query and confirm source_drawer_id appears in the response.

Notes

• adapter_name is not exposed at the MCP boundary in this PR. Upstream only sets adapter_name from inside adapter code that advertises supports_kg_triples; exposing it at the MCP boundary would let arbitrary callers spoof adapter identity. The column exists at the storage layer and is plumbed through Triple, but the MCP tool only forwards source_drawer_id.
• The internal add_triple test call sites were padded mechanically (all existing calls used 8 args; the two new kwargs default to None). No test semantics changed.
• Diff size: 3 files, +331/-16 lines. No new dependencies.