fix(memory): unify Outcome enum and add ABANDONED support

- Add ABANDONED value to core Outcome enum in types.py
- Replace duplicate OutcomeType class in mcp/tools.py with alias to Outcome
- Simplify mcp/service.py to use outcome directly (no more silent mapping)
- Add migration 0006 to extend PostgreSQL episode_outcome enum
- Add missing constraints to migration 0005 (ix_facts_unique_triple_global)

This fixes the semantic issue where ABANDONED outcomes were silently
converted to FAILURE, losing information about task abandonment.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/alembic/versions/0006_add_abandoned_outcome.py

@@ -0,0 +1,52 @@
+"""Add ABANDONED to episode_outcome enum
+
+Revision ID: 0006
+Revises: 0005
+Create Date: 2025-01-06
+
+This migration adds the 'abandoned' value to the episode_outcome enum type.
+This allows episodes to track when a task was abandoned (not completed,
+but not necessarily a failure either - e.g., user cancelled, session timeout).
+"""
+
+from collections.abc import Sequence
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "0006"
+down_revision: str | None = "0005"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    """Add 'abandoned' value to episode_outcome enum."""
+    # PostgreSQL ALTER TYPE ADD VALUE is safe and non-blocking
+    op.execute("ALTER TYPE episode_outcome ADD VALUE IF NOT EXISTS 'abandoned'")
+
+
+def downgrade() -> None:
+    """Remove 'abandoned' from episode_outcome enum.
+
+    Note: PostgreSQL doesn't support removing values from enums directly.
+    This downgrade converts any 'abandoned' episodes to 'failure' and
+    recreates the enum without 'abandoned'.
+    """
+    # Convert any abandoned episodes to failure first
+    op.execute("""
+        UPDATE episodes
+        SET outcome = 'failure'
+        WHERE outcome = 'abandoned'
+    """)
+
+    # Recreate the enum without abandoned
+    # This is complex in PostgreSQL - requires creating new type, updating columns, dropping old
+    op.execute("ALTER TYPE episode_outcome RENAME TO episode_outcome_old")
+    op.execute("CREATE TYPE episode_outcome AS ENUM ('success', 'failure', 'partial')")
+    op.execute("""
+        ALTER TABLE episodes
+        ALTER COLUMN outcome TYPE episode_outcome
+        USING outcome::text::episode_outcome
+    """)
+    op.execute("DROP TYPE episode_outcome_old")