feat(PRLT-1271): list available sessions when orchestrator --name doesn't match

[chrismcdermut]

Summary

• When prlt orchestrator attach --name X (or stop --name X) doesn't match a running session, the error now lists the available sessions instead of a generic "not running" message.
• Adds a small formatAvailableSessions helper rendering "main (/Projects/backend), backend (/Projects/proletariat-hq), reviewer (host)" with Docker tagging.
• The picker, JSON-mode prompt config, and grouped status (PRLT-1329) were already in place; this fills the last gap from PRLT-1271.

Test plan

• pnpm build (apps/cli)
• npx mocha test/unit/orchestrator-format.test.ts — 5/5 passing
• npx mocha test/unit/orchestrator-attach-scope.test.ts — 8/8 passing (no regression)

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