Features:
- Exclude output tokens from conversation size calculation for more accurate context tracking
- Add -contextsize flag to print the current conversation size from the last assistant message
- Update installation path to use Go's default bin directory (GOPATH/bin or GOBIN)

Changes:
- TotalTokens now represents only tokens that count towards the context (input, cache read, cache creation)
- Output tokens are still displayed separately but not included in the conversation size total
- Makefile install target now respects Go's installation conventions