fix: improve local command discovery

[chaim0m]

Summary

• make dci version a first-class local command instead of interpreting version as an API hostname
• keep root help compact by showing common commands and explicit discovery paths
• return concise unknown-command errors with close-match suggestions and exit code 2
• emit one UNKNOWN_COMMAND JSON envelope in agent mode
• advertise the complete machine-readable catalog when dci commands is available

Test methods

Automated validation run on this branch:

go test ./...
go vet ./...

Manual validation:

go build -o /tmp/dci-pr36 .
/tmp/dci-pr36 version
/tmp/dci-pr36 --help
/tmp/dci-pr36 list-bugets
echo $?
DCI_AGENT_MODE=1 /tmp/dci-pr36 list-bugets
echo $?

version should return locally without a network lookup. Root help should remain short. The typo should suggest list-budgets, avoid printing the complete catalog, and exit 2; agent mode should emit one JSON error.

Could this break things?

Risk: low-medium. Scripts that scrape the former very long root help, or assert the old unknown-command text, will see a change. Successful API commands are untouched. Treating version as a local command fixes the current accidental request to https://version; only an unusual workflow that intentionally used version as a host token would behave differently.