Implement Phase 1: core CLI with exercises, templates, sessions, and … #1

Job	Run time
binaries (bun-windows-x64, windows-x64, zip)	17s
binaries (bun-linux-arm64, linux-arm64, tar.gz)	15s
binaries (bun-darwin-x64, darwin-x64, tar.gz)	11s
binaries (bun-darwin-arm64, darwin-arm64, tar.gz)	11s
binaries (bun-linux-x64, linux-x64, tar.gz)	12s
release	9s
	1m 15s

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