Fix materialize dry-run filesystem writes

[RichardHightower]

Summary

• make pkc_materialize --dry-run compute create/update decisions without writing concepts or directories
• suppress write events, catalog refreshes, and log.md changes during previews
• distinguish planned output with would_create / would_update, planned: true, and an explicit text-mode dry-run banner
• add a byte-for-byte target-tree regression covering concepts, indexes, log.md, and write-events/
• capture and complete the required repository worklog plan

Fixes #69

Verification

• python3 tests/test_pkc.py — 98 tests passed
• python3 scripts/pkc_validate.py --bundle sample-knowledge — 0 errors (8 existing missing-optional-catalog warnings)
• git diff --check