release v0.3.0: Chat zero-value safety and consistent constructors

[x2d7]

Summary

Two related improvements to Chat initialization and API consistency.

Fix: Chat panics without Tools field (#30)

Chat{} without Tools caused a nil pointer panic on the first Session() call.
ensureDefaults() now initializes missing Messages and Tools lazily at the entry point — existing values are never overwritten.

Refactor: consistent constructors

NewTools() previously returned Tools by value, while NewMessages() returned *Messages — forcing callers to manually take &toolList. Both types contain a mutex internally, so returning by pointer is correct regardless. Now both constructors return a pointer.

// before
toolList := tools.NewTools()
c := chat.Chat{Tools: &toolList}

// after
c := chat.Chat{Tools: tools.NewTools()}

Closes #30