fix: consistent fetch --max-amount cap error message

[reneaaron]

What

fetch --max-amount validates its denomination through the shared amount classifier (src/amount.ts), which speaks the whole CLI's vocabulary — fiat codes, chain networks, and a <BTC|USD|EUR|USDC|…> hint. For an invalid cap, those raw classifier errors leaked through, e.g.:

$ fetch <url> --max-amount 100 --currency USD --unit sats
{ "error": "An amount requires --network <name>. Use \"lightning\" ... Use a chain name (e.g. arbitrum) ..." }

$ fetch <url> --max-amount 100
{ "error": "An amount requires --currency <BTC|USD|EUR|USDC|…> so the denomination is never guessed" }

Both advertise rails fetch's cap doesn't accept (fiat, arbitrum, USDC) and contradict the command's own help, which says the cap requires --currency BTC --unit sats --network lightning.

Fix

Wrap the classifyRail call in fetch so any rejection — or any non-BTC/sats rail it classifies — collapses into fetch's single supported-rail message:

{ "error": "fetch's --max-amount spend cap currently supports only --currency BTC --unit sats --network lightning" }

Valid caps behave exactly as before; only the error text for invalid ones changed. The shared classifier and other commands (pay, etc.) are untouched.

Test

• yarn test green (110 pass, 2 skipped).
• New table test in fetch-max-amount.test.ts asserts the four previously-leaking combos (no denomination / fiat+unit / BTC without network / token on a chain) all surface fetch's own message and no longer leak arbitrum or the BTC|USD|EUR|USDC vocabulary.

Context

Found while consumer-testing the CLI for #49. Separate from that PR since it's a pre-existing issue independent of the discover/bridge work.

Not included: the -v/--verbose flag being quiet during fetch — that turned out to be by-design (verbose is a connection-flow flag, same as pay), not a bug.

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