feat: rethink credential storage and reviewer entity init UX

[rianjs]

Summary

• move credential storage, LLM runtimes, and reviewer entities toward first-class reusable init concepts
• remove the user-facing/default-profile dependency from the init workflow
• make reviewer entities and LLM runtimes configurable before review profiles
• align init inventory screens with the linear action-list UI language
• bump minor version to 0.7

Verification

• rtk go test ./internal/config ./internal/configedit ./internal/identity ./internal/cmd/configcmd ./internal/cmd/mecmd ./internal/cmd/credentialcmd
• rtk go test ./...
• tmux smoke checks for root menu, secrets storage, LLM runtime inventory, reviewer entity inventory, reviewer PAT detail, and review profile create flow
• rtk rg -n "default_profile|default profile|DefaultProfile|defaultProfile" .