Add CLI error-scenario smoke test matrix for devs

Description

Manually verifying CLI error scenarios across all commands is slow and ad-hoc. During PR #468 (CliFatal sentinel work) we hand-rolled a 15-scenario bash sweep to confirm error output and exit codes. We should turn this into a reusable script or test suite so future CLI changes can re-run the matrix in one command.

Goals

• Cover every command + every documented error scenario (invalid args, missing flags, bad RPC, no auth, insufficient funds, user cancel, etc.)
• Capture stdout, stderr, exit code per scenario
• Run without live RPC where possible (use mocks or no-network failure paths)
• Clear pass/fail per scenario; easy to add new scenarios

Acceptance criteria

• pnpm run smoke (or similar) executes the full matrix
• Output format makes it easy to spot regressions (snapshot diff or labeled per-scenario block)
• At minimum covers: payments setup/fund/deposit/withdraw/status, add, import, data-set show/list/terminate, provider list/show/ping, rm
• Argument-validation scenarios (no network, no auth) run in CI
• Network-dependent scenarios (insufficient funds, bad RPC) are clearly tagged and skipped in CI by default
• README or CONTRIBUTING.md documents how to run + add scenarios

Notes

• See PR #468 conversation for the manual sweep template
• Reuse existing src/test/mocks/synapse-sdk.js where mocking is preferable to live RPC
• Consider snapshot testing on captured stderr (vitest toMatchInlineSnapshot)
• Out of scope: replacing existing unit/integration tests