Feat/add export snapshot

[111r1ck]

What does this PR do?

• add a new mempalace export <output_dir> CLI command
• support --snapshot-name and --wing for snapshot exports
• generate overview.md, manifest.json, root index, and per-wing indexes
• preserve the existing verbatim room markdown export format
• add implementation-focused docs for the snapshot export feature
• stabilize Windows/offline test coverage for onboarding and convo mining verification

This PR adds a first-class snapshot export workflow for MemPalace.

The new export path builds on the existing markdown exporter and adds a human-readable, archive-friendly snapshot layout without changing storage behavior or drawer content. Snapshot exports remain additive and preserve the verbatim guarantee.

The implementation includes:

• CLI wiring in mempalace/cli.py
• snapshot export generation in mempalace/exporter.py
• CLI and exporter test coverage
• doc cleanup so repository docs keep only implemented snapshot behavior

This PR also includes test-only reliability fixes needed to keep full-suite verification green in the current Windows/offline environment:

• explicit UTF-8 reads in onboarding bootstrap tests
• offline deterministic embedding use in test_convo_mining

How to test

• python -m pytest tests -v
• python -m ruff check mempalace/exporter.py mempalace/cli.py tests/test_exporter.py tests/test_cli.py
• python -m ruff check tests/test_onboarding.py tests/test_convo_miner.py

Checklist

• Tests pass (python -m pytest tests -v)
• No hardcoded paths
• Linter passes (ruff check)