docs: add buddy preview assets and usage notes

docs(research): add claude code feature index

docs: add login-preserving claude alias with IS_DEMO masking

docs: fix root README links and add recovered tree overview

docs: sync full Chinese system prompt analysis into dated note

docs: use relative source links in research notes

docs: index today's research in root README and split dated topic notes

chore: add current codex session jsonl for study

chore: import full claude-code-source workspace and recovered sources