Stitch continued session transcripts in WebUI

[Jellypowered]

This fixes a WebUI history issue where sessions split by continuation events (such as compression or CLI-close) could show only the latest segment instead of the full conversation.

What changed

• get_cli_session_messages() now walks valid continuation lineage and stitches messages across session segments in chronological order
• /api/session now merges sidecar history with CLI lineage history instead of blindly preferring only one source
• added regression coverage for the session API behavior around stitched/merged history

Why

Some continued sessions were visibly fragmented in the UI even though they were still part of the same logical conversation. This made it difficult to read the full transcript when the session had multiple continuation segments.

Notes

• continuation stitching only applies to real continuation boundaries, not arbitrary child sessions
• merged transcript results are deduplicated to avoid repeated messages when both sources contain overlapping history

Test Plan

• source /root/.hermes/hermes-webui/.venv/bin/activate && python -m pytest -q /root/.hermes/hermes-webui/tests/test_session_lineage_full_transcript.py /root/.hermes/hermes-webui/tests/test_cli_session_tool_metadata.py /root/.hermes/hermes-webui/tests/test_session_lineage_metadata_api.py
• result: 8 passed in 3.97s

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This PR was prepared with assistance from hermes-agent.