feat(02-04): wire escalation into orchestrator pipeline

- Add escalation pre-check in _process_message: assistant mode for escalated threads
- Add escalation post-check after LLM response: calls escalate_to_human on rule match
- Load Slack bot token unconditionally (needed for escalation DM, not just placeholders)
- Add keyword-based conversation metadata detector (billing keywords, attempt counter)
- Add no-op audit logger stub (replaced by real AuditLogger from Plan 02 when available)
- Add escalation_assignee and natural_language_escalation fields to Agent model
- Add Alembic migration 003 for new Agent columns

migrations/versions/003_escalation_fields.py

@@ -0,0 +1,56 @@
+"""Phase 2 Plan 04: Add escalation_assignee and natural_language_escalation to agents
+
+Revision ID: 003
+Revises: 002
+Create Date: 2026-03-23
+
+These columns support the human escalation/handoff system:
+- escalation_assignee: Slack user ID of the human to DM when escalation triggers
+- natural_language_escalation: tenant-level flag enabling NL phrase detection
+
+Both are nullable/defaulted so existing agent rows are unaffected.
+"""
+
+from __future__ import annotations
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = "003"
+down_revision: Union[str, None] = "002"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add escalation_assignee column — nullable TEXT
+    op.add_column(
+        "agents",
+        sa.Column(
+            "escalation_assignee",
+            sa.Text,
+            nullable=True,
+            comment="Slack user ID of the human to DM on escalation (e.g. U0HUMANID)",
+        ),
+    )
+
+    # Add natural_language_escalation column — NOT NULL with default False
+    op.add_column(
+        "agents",
+        sa.Column(
+            "natural_language_escalation",
+            sa.Boolean,
+            nullable=False,
+            server_default=sa.text("FALSE"),
+            comment="Whether natural language escalation phrases trigger handoff",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("agents", "natural_language_escalation")
+    op.drop_column("agents", "escalation_assignee")