fix: bare selat fund is not help; selat-pay packageRoot ignores ancestor packages

[SELAT-DEV]

Closes #191. Closes #192.

QA residuals after Arc raw-key cleanup (#190 / discovery#170), outside the key-blind matrix.

Root cause

#191 — selat fund help guard. Help must be inert (-h/--help print usage, exit 0, never enter the deposit flow) and must not swallow a bare command. The guard is exact -h / --help argv tokens only. Empty argv is the deposit path: non-TTY still errors with the --amount / no TTY prompt, not Usage: selat fund.

#192 — packageRoot override resolve. findPackageRoot walked up from $SELAT_PAY_BIN and returned the first package.json it saw. That is wrong for a bare override (a file with no adjacent selat-pay package): an in-tree TMPDIR, or any nested temp under another package, resolved to that ancestor (here, selat-cli) and selatPayVersion() claimed its version. Env-override with a real @selat-ai/selat-pay layout (<root>/bin/selat-pay.mjs) still walks up to that package.

What changed

• lib/commands/fund.mjs: wantsFundHelp() matches only exact -h/--help tokens; empty argv is never help.
• lib/selat-pay.mjs: findPackageRoot only accepts package.json whose name is @selat-ai/selat-pay; unrelated ancestors are skipped.
• test/selat-pay-resolve.test.mjs: the bare-override case nests under a decoy @selat-ai/selat-cli package.json so a naive walk cannot pass when TMPDIR is /tmp.

Test evidence

node --test test/fund-help-guard.test.mjs test/selat-pay-resolve.test.mjs
# 12/12 pass (default TMPDIR and TMPDIR=/workspace)

npm test
# 597/597 pass

Includes:

• bare fund (no --help) does NOT print help — still enters its own flow
• fund --help / -h print usage, exit 0, no deposit flow
• a bare override resolves with a null packageRoot and no version claim
• env-override with a real package root still walks up correctly