Improve session export timeline and usage visibility

[MonishGosar]

Motivation

Make session HTML exports easier to scan and compare during long coding sessions.

Changes

• Add compact session summary cards for entries, messages, tools, events, and duration.
• Add a clickable timestamp timeline with message, tool, and event lanes.
• Preserve and test rendered ThinkingContent blocks.
• Clarify that Usage charts and cost estimates are export-time snapshots.
• Update the sessions guide and Phase 20.4 architecture notes.

Validation

• py -3.13 -m pytest tests/test_session_export.py tests/test_session_usage.py -q — 26 passed
• py -3.13 -m ruff check .
• py -3.13 -m ruff format --check .
• git diff --check

The generated HTML previews were used for manual inspection and are intentionally not included in the commit. Hugo was unavailable locally, so the docs site build should be verified by CI.