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)

[igorls]

Hi, thanks for the contribution.

This PR has merge conflicts with develop, and the branch has not been updated in over 7 days, which puts it before our most recent release. The conflicts are likely against work that landed in that release.

Could you rebase onto develop so we can take another look?

If this change is no longer relevant, feel free to close the PR.

(This message is part of a periodic backlog pass, sent to all open PRs that match this state.)

[111r1ck]

Rebased onto latest develop and force-pushed. Conflicts are resolved, ready for re-review.