sessions-graph: reconcile_session() now writes episodic Session.summary

[antejavor]

Summary

• reconcile_session() extracted entities from session content but wrote nothing that could answer "what happened last time?" -- actions-graph's raw Action/FOLLOWED_BY capture is already fully structured, but nothing distilled it into a narrative gist.
• Entity extraction and narrative summarization are different task shapes (LightRAG's extraction prompt isn't a summarizer), so this adds a second, dedicated LLM call using the same configured llm_model_func, over the same deduped session text already fetched for entity extraction. Still one reconcile_session() pass, one trigger, no separate schedule -- just a second internal LLM call.
• Writes an (:Episode {summary, summarized_at}) node, linked via (:Session)-[:HAS_EPISODE]->(:Episode) -- MERGEd on the relationship pattern so re-reconciling a session updates its one Episode instead of accumulating duplicates.
• ReconciliationSummary gains summary_written so callers (and the CLI) can tell whether an Episode was produced.
• Updates sessions-graph/README.md's "Session reconciliation" section accordingly.

Implements the episodic-memory design from Grilling: what does continuous episodic-memory organization mean?, a child ticket of the Wayfinder map Continuous memory organization for context-graph -- note: #261 originally decided on a summary property directly on (:Session), no new node. That was reversed after further discussion during implementation: episodic memory is a real Episode node instead, matching this codebase's existing pattern of dedicated node types for owned data (Memory, Action, Chunk) rather than growing Session's own property set indefinitely. See the ticket for the full history.

Test plan

• uv run --package sessions-graph --extra test --extra agent-context-graph --extra reconciliation pytest context-graph/sessions-graph/tests/ -q (excluding live-key e2e) -- 54 passed
• ruff check / ruff format --check on changed files
• Live e2e run against a real Memgraph container + real OpenAI API key (test_e2e_reconciliation.py, extended with Episode assertions) -- confirmed real entity extraction and a real Episode node/HAS_EPISODE edge from one reconcile_session() call
• Re-ran reconcile_session() a second time for the same session against the live container -- confirmed exactly one Episode still exists (MERGE, not CREATE, is working as intended)