docs: add repo work ledger

[unityequilibrium]

Summary

• Replaces the narrow work-log rule with a repo-wide WORK_LEDGER policy in AGENTS.md
• Adds WORK_LEDGER/README.md, AREAS.md, and TEMPLATE.md for daily cross-area progress logging
• Converts the old docs/UET_Documentation_Details/WORK_LOG/README.md into a pointer to WORK_LEDGER so there is only one daily ledger location

Scope

Covers research, theory/history, book writing, Thailand proposals, services/tools, result artifacts, repo ops, and raw/private source handling.

Safety

• Branch is built from origin/main with an isolated Git index
• Does not include local dirty workspace, chat archives, raw assets, transcript files, or cache files