Add global CLI preferences

[Icemap]

Summary

• add the optional hidden ~/.tdc/.preferences TOML file for process-wide logging and future telemetry preferences
• move operation logging configuration out of profile-scoped ~/.tdc/config, with strict parsing, fail-closed behavior, and idempotent legacy migration
• reserve the logging profile name and keep every tdc update form isolated from all ~/.tdc/ state
• move the completed global-settings spec to done/ and renumber the remaining telemetry and distribution specs
• synchronize README, product principles, AGENTS guidance, and the PingCAP documentation PR

Why

Global logging and telemetry preferences are not profiles. Keeping [logging] in the profile document made profile persistence responsible for preserving unrelated global state and left future telemetry configuration without a clear ownership boundary. The hidden preferences file makes advanced global controls explicit without adding another public command.

User Impact

Users can persistently configure global logging in ~/.tdc/.preferences or override it for one process with TDC_LOGGING. Existing [logging] values in ~/.tdc/config migrate automatically. Fresh installs and tdc configure do not create the preferences file, and malformed preferences never fail the requested command.

Related documentation: pingcap/docs#23306

Validation

• make test
• make e2e
• git diff --check
• PingCAP docs git diff --check