feat(cli): add mempalace export subcommand

[chrischen1104]

Summary

Wires the existing mempalace.exporter.export_palace() to argparse as a new top-level subcommand, so users can dump the palace as a browsable Markdown tree without writing Python:

mempalace export <output_dir>

The exporter module itself is unchanged — this is pure CLI plumbing.

What changed

• mempalace/cli.py — new cmd_export handler + subparser registration + dispatch entry, mirroring the existing cmd_search style
• tests/test_cli.py — 3 unit tests (handler arg forwarding, ~ expansion, main() dispatch routing) following the established cmd_* test pattern
• CHANGELOG.md — one bullet under [3.3.4] — unreleased
• website/reference/cli.md — reference entry placed adjacent to mempalace status (both are read-only palace-inspection commands)

Test plan

• python -m pytest tests/test_cli.py -v -k export — 3 new tests pass
• python -m pytest tests/ -v --ignore=tests/benchmarks — full suite still green
• ruff check mempalace/cli.py tests/test_cli.py && ruff format --check mempalace/cli.py tests/test_cli.py
• Manual end-to-end: mempalace init <dir> && mempalace mine <dir> && mempalace export /tmp/out → /tmp/out/index.md + per-wing dirs created

Out of scope

• No changes to exporter.py itself — its existing test coverage in tests/test_exporter.py remains the source of truth for export behaviour.
• No README change — README's Quickstart deliberately lists only the 4 daily-driver commands (init, mine, search, wake-up); export belongs with compress/repair/migrate in the auxiliary tier (CLI reference + --help only). Happy to add it to the Quickstart if maintainers prefer.
• No new MCP tool — CLI-only for now; can be added in a follow-up if there's demand.

[mvalentsev]

Heads-up -- overlap with #1086 (jphein, opened 2026-04-21) which has the same goal: wire mempalace.exporter.export_palace() to argparse as mempalace export. Same shape (CLI plumbing only, exporter.py untouched), same touched files (mempalace/cli.py + tests/test_cli.py), same from .exporter import export_palace + same export_palace(palace_path=..., output_dir=...) call. #1086 shows mergeStateStatus: CLEAN against develop.

Differences are cosmetic: #1086 takes the dir as -o <dir> flag, this PR takes it as a positional. This PR also adds a website/reference/cli.md entry, which #1086 doesn't.

Two things worth noting before review:

1. Base branch: this PR targets main, but the active branch is develop -- main is the release-tag branch and only moves on prep merges. mergeStateStatus: BEHIND reflects that. Compare with feat(cli): wire export_palace() to CLI as mempalace export #1086 -> develop.

2. Related (broader, separate scope): Feat/add export snapshot #775 (111r1ck) is also in this area but modifies exporter.py itself (adds snapshot functionality) -- a superset, currently UNSTABLE.