fix(hitl): recognize real Signal account listings

[lalalune]

Follow-up to merged #15760 for #14797.

Reviewing the Signal linked-account change against the current upstream CLI exposed two contract mismatches:

• dashboard availability used only listAccounts exit status, but the upstream command returns normally for an empty account collection, so an unlinked CLI could be reported ready;
• the live probe accepted only bare +number/u:username lines, while current signal-cli plaintext output is Number: <account>, so a linked account could be reported absent.

The upstream implementation is visible in ListAccountsCommand.java.

This adds a declarative command-output-nonempty availability leaf, carries stdout through the real-machine evaluator, requires actual account-list output, and normalizes the upstream Number: prefix before validating and masking account identifiers.

Verification

• focused registry/probe suite: 26/26 passed;
• complete test:lifeops-hitl lane: 58/58 passed;
• Biome passed on all four changed scripts;
• git diff --check passed;
• tests cover a zero-exit empty listing, current-format non-empty output, live-probe parsing, and last-four-only masking.

• Original pre-change Signal dashboard screenshots

• Current Signal dashboard screenshots from the parent change

• Parent Signal desktop/mobile dashboard walkthrough

• Upstream contract review and 58-test verification

• Parent current-branch desktop console log

• N/A - no agent action, prompt, provider, model, or trajectory behavior changed.

• N/A - read-only account enumeration sends no Signal messages and writes no LifeOps domain records.

• Parent current-branch OCR review manifest

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