WS-B.3: exitcodes.go named constants (138 sites) + cross-command exit-code table in docs

[LukasWodka]

Part of epic tracebloc/backend#1106. Full spec: report §5 (mirrored on the epic; artifact https://claude.ai/code/artifact/b4b6e6a6-4d76-4e46-a668-6f4f0f944344). WS-B PR3: internal/cli/exitcodes.go naming the 138 non-test &exitError{code:N} sites (63x1,27x2,24x3,9x4,4x5,1x6,7x7,2x8,5x9,1x0); exit.go already centralizes extraction — naming sweep, not redesign. Add the cross-command exit-code table to docs/troubleshooting.md (today only data ingest --help documents them). Customers script against these codes.

[LukasWodka]

Shipped in #315 (merged to develop). Closing as completed — CLI refactor epic backend#1106 cleanup.