fix(convos): honor mined state during dry runs

[fatkobra]

What does this PR do?

• open an existing palace with create=False and read-only intent during conversation dry-runs
• prefetch the same source-mtime and content-hash state used by a real mine
• apply the unchanged-source skip check in dry-run mode
• treat a missing palace or collection as empty prior state without creating it

Root cause

_mine_convos_impl() set collection to None during dry-runs, built empty deduplication maps, and gated _is_unchanged_since_last_mine() behind not dry_run.

A dry-run therefore could not make the same unchanged-source decision as the subsequent real run.

Tests

• an unchanged, previously mined transcript is skipped during a dry-run, including when a different target wing is passed
• a modified transcript remains visible as pending work
• a dry-run against a missing palace reports pending work without creating the palace directory

Closes #2118

How to test

Run

python3 -m ruff format mempalace/convo_miner.py tests/test_convo_miner.py
python -m ruff format --check .
python -m ruff check .
python -m pytest tests/test_convo_miner.py \
  -k "dry_run_skips_unchanged_mined_file or dry_run_keeps_modified_file_as_work or dry_run_missing_palace_does_not_create_it" \
  -q
python -m pytest tests/test_convo_miner.py -q
python -m pytest tests/ -v

Checklist

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