Closes #32.

No more legacy. All should be canonical.

Issue #32 asked for sphere-cli UX consistency across five concrete goals.
This PR delivers all five passes as a clean break — no backwards-compat
shims for the legacy quoted-asset form or for the JSON-by-default output.

What changed

Pass A — output formatting

• New formatOutput(payload, shape, label?) helper with a typed
  OutputShape union and per-shape renderers (identity, invoice-terms,
  invoice-status, transfer-result, swap-status, swap-ping, auto-return,
  invoice-transfers, generic, …).
• Global --json flag opts back into JSON; default is a human-readable
  labelled block.
• Replaced 22 console.log(JSON.stringify(...)) sites. File-write /
  POST-body JSON.stringify calls remain (wire format).

Pass B — canonical asset input

• Dropped parseAssetArg (quoted compound form, "10 UCT").
• New consumeAssetPair(args, startIdx) reads two argv tokens.
• invoice-create, invoice-return, swap-propose now take
  --asset <amount> <coin>, --offer <amount> <coin>, --want <amount> <coin>.
• invoice-create supports multiple --asset flags for multi-asset
  invoices.

Pass C — --help works for every command/subcommand

• Early-dispatch shim in main() catches --help / -h before the
  switch, handling sub-subcommand keys (wallet create, daemon start)
  via compound lookup with fallback to top-level.
• src/index.ts: legacy subcommands disable commander's built-in
  --help (helpOption(false)) so the flag reaches our dispatcher.
• help added to LEGACY_NAMESPACES so sphere help <cmd> routes
  through the COMMAND_HELP registry.
• Universal flags (--json, --help/-h) shown in every help block.

Pass D — completion + docs

• Added COMMAND_HELP entries for completions and help so the
  drift-guard test passes.
• README: new "UX — canonical conventions" + "Shell completion"
  sections documenting the canonical syntax and three install paths
  (no-sudo, system-wide, per-user zsh fpath).

Pass E — auto-help on invalid input

• Single failWithHelp(cmdName, errorMsg) helper prints
  Error: <msg> followed by the full COMMAND_HELP block to stderr,
  then exit(1).
• Replaced ~40 console.error('Usage: …'); process.exit(1) sites.
• New resolveInvoiceId() helper collapses the duplicated
  invoice-prefix lookup boilerplate across ~10 invoice-* commands.

Static UX guard

New src/legacy/legacy-cli-ux.test.ts enforces the invariants so
future drift surfaces as a test failure:

1. No stray console.log(JSON.stringify(...)) in the dispatch body.
2. No console.error('Usage: …'); process.exit(…) legacy stubs.
3. No parseAssetArg(...) call sites.
4. Every COMMAND_HELP key is reachable through the completion list.
5. Helpers (formatOutput, failWithHelp, consumeAssetPair) exist.
6. --json and --help shims are wired in main().

Verification

npx tsc --noEmit            # clean
npx vitest run              # 113/113 pass (incl. 7 new UX guard tests)
npx tsup                    # build clean

Smoke-tested in dev:

• sphere invoice create --help → prints the new canonical help block
• sphere swap propose --help → canonical --offer <amount> <coin> form
• sphere help send → routes through COMMAND_HELP['send']
• sphere payments send (no args) → Error: … + full help block + exit 1
• sphere wallet init integration test compatibility: human render
  keeps l1Address/directAddress/chainPubkey as labels so the
  existing toMatch(/l1Address/) assertion still passes.

Breaking changes (intentional — "no more legacy")

• The quoted compound asset form (--asset "10 UCT") is no longer
  accepted. Use --asset 10 UCT (two positional tokens).
• Default output is human-friendly, not raw JSON. Scripts that scrape
  CLI stdout must pass --json.

Both are explicitly called out in the new "UX — canonical conventions"
README section so the migration path is one place.

Out of scope (per the issue)

• Cosmetic redesign (colors, ASCII art).
• Migrating off the legacy-cli.ts monolith — that's its own refactor;
  this PR lands within the existing file structure as the issue
  permits.
• Daemon-mode command output formatting (separate UX surface).