refactor(cli): migrate commands to RunE with a single exit funnel

[Caesarsage]

depend on #489

Part of the Microcks CLI v2 work (#255). Stacked on PR #489.

• All ten commands move from Run + scattered os.Exit/log.Fatal to RunE returning errors.
• A single funnel — cmd.Handle, called only by main — prints to stderr and maps Failure Kind → exit code (cmd/exit.go). Nothing else exits.
• A non-conforming contract test travels as the silent ErrTestFailed sentinel → exit 1 with no spurious error line (mirrors kubectl diff).

No impact on the microcks binary: commands, flags, args, and success output are unchanged. Only code importing these packages is affected.

Exit codes (new, user-visible)

0 = success / non-zero = failure still holds, and 1 still means "contract test failed". Operational failures now carry distinct codes so CI can branch on why:

Code	Meaning
0	success / contract conformed
1	contract test failed (a clean run)
2	usage — bad arguments or flags
11	connection — couldn't reach Microcks/Keycloak
12	API — server rejected the request / unusable response
13	not-found — remote resource missing
14	environment — local precondition (container runtime, image, readiness)
20	generic — unclassified failure

These codes were previously undocumented and inconsistent (1/2/20 interchangeably), so there is no prior contract to break. Documented in README.md + documentation/error-handling.md.

[Caesarsage]

HOLD

till PR #489 is review and merged