feat(tasks): phase 1 — elevate chat session to a task (schema + task entity)

Migration 018 adds goal/status/outcome/summary/entity_id to agent_sessions
and creates session_plan_steps + session_questions. Registers a 'task'
entity type and an 'involves' (task→entity) relationship in the ontology
so each session anchors its knowledge and involved-entity edges on the
existing relationships graph.

nomos createSession now mints a task:<session-id> entity (type task) and
links it via agent_sessions.entity_id — best-effort so chat never blocks on
it. listSessions/GET /sessions surface the new task fields.

No behaviour change yet; this is the data foundation for the task board and
live context panel. Verified end-to-end against the local stack: migration
applied, ontology ingested (60 types/47 rels), a new session mints a linked
task entity and the API returns status/goal/entity_id.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

migrations/018_tasks.up.sql

@@ -0,0 +1,53 @@
+-- 018_tasks.up.sql
+-- Elevate a chat session into a "task": a goal-structured unit of work with a
+-- lifecycle status, an outcome, and a one-line summary — the first-class object
+-- the task board and the live context panel render. See
+-- plans/2026-07-11-goal-oriented-chat-control-panel.md.
+--
+-- entity_id links the session to its OWN entity (type 'task', registered in
+-- seeds/ontology.yaml) so knowledge notes and involved-entity edges hang off
+-- the existing relationships graph unchanged — get_relations and
+-- get_entity_knowledge just work. Intentionally no hard FK (mirrors 017's
+-- decoupling): a race between task-entity creation and the session insert must
+-- not be able to break the session.
+
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS goal      TEXT NOT NULL DEFAULT '';
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS status    TEXT NOT NULL DEFAULT 'active';
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS outcome   TEXT;
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS summary   TEXT NOT NULL DEFAULT '';
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS entity_id UUID;
+
+-- Ordered plan steps. A step is a described unit of work that maps to a
+-- run/request_execution call (no fixed step enum, per general-gated-execution).
+-- execution_id is the gated action a step runs, if any; its terminal status
+-- auto-closes the step server-side.
+CREATE TABLE IF NOT EXISTS session_plan_steps (
+    id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    session_id   UUID NOT NULL REFERENCES agent_sessions(id) ON DELETE CASCADE,
+    seq          INT  NOT NULL,
+    title        TEXT NOT NULL,
+    detail       TEXT NOT NULL DEFAULT '',
+    status       TEXT NOT NULL DEFAULT 'pending',
+                 -- pending | running | done | failed | skipped | blocked
+    execution_id UUID,
+    target_slug  TEXT,
+    started_at   TIMESTAMPTZ,
+    finished_at  TIMESTAMPTZ,
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+CREATE INDEX IF NOT EXISTS idx_plan_steps_session ON session_plan_steps(session_id, seq);
+
+-- Structured decisions the agent surfaces to the operator mid-task. context
+-- carries { entities:[], options:[], why:"" } for the pinned question card.
+CREATE TABLE IF NOT EXISTS session_questions (
+    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    session_id  UUID NOT NULL REFERENCES agent_sessions(id) ON DELETE CASCADE,
+    prompt      TEXT NOT NULL,
+    context     JSONB NOT NULL DEFAULT '{}',
+    status      TEXT NOT NULL DEFAULT 'open',   -- open | answered | dismissed
+    answer      TEXT,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT now(),
+    answered_at TIMESTAMPTZ
+);
+CREATE INDEX IF NOT EXISTS idx_questions_session_open
+    ON session_questions(session_id) WHERE status = 'open';