Refactor: extract shared mv/cp relocation driver

[AndreyVMarkelov]

Summary

mv and cp had ~115 lines of near-identical body — the same two-phase validate/execute loop, destination resolution, skip and conflict handling, dry-run preview, and JSON accumulation — differing only in a handful of values (command name, verb, success status, missing-args message, and which API call to make).

This introduces a relocationSpec describing those differences and a single runRelocation driver in cmd/relocation.go. mv and cp become one-line delegations:

func mv(cmd *cobra.Command, args []string) error { return runRelocation(cmd, args, moveSpec) }
func cp(cmd *cobra.Command, args []string) error { return runRelocation(cmd, args, copySpec) }

No behavior change. All command-specific strings are preserved via the spec, including mv's distinct "mv command requires…" vs cp's "cp requires…" missing-args wording.

Net: 4 files, +367/−229 (cp.go −116, mv.go −115, relocation.go +168, relocation_test.go +197).

Tests

New cmd/relocation_test.go exercises runRelocation over both specs (table-driven mv/cp):

• distinct missing-args message per command
• dry-run source-lookup, destination-check, and result-build error branches — each asserting the correct verb and (for dry-run) that no API is called
• Move-vs-Copy API routing with the correct success status

Raises runRelocation coverage from 85% → 97%. The only uncovered block is a pre-existing unreachable validation branch (makeRelocationArg — validatePath never errors), carried over verbatim; a candidate for separate cleanup.