fix(channel): reject missing member lists

[andrei-hasna]

Summary

• return a structured CLI not-found error for channel members <missing> --json
• return structured JSON 404s for missing-channel member-list requests on both hosted /api and authenticated /v1
• preserve successful empty lists for channels that exist without members

Verification

• focused CLI regression: 2 pass, 0 fail
• focused API regression: 1 pass, 0 fail
• hosted API regression: 2 pass, 0 fail
• full hosted serve affected lane: 69 pass, 0 fail
• initial candidate full suite: 1790 pass, 0 fail; remediation revalidation scoped to the affected hosted/CLI/API lanes above
• typecheck: pass
• build: pass
• staged and exact-commit gitleaks: no leaks found
• independent adversarial review: focused remediation GO; no reachable in-scope P0/P1 defects

Provenance

• task: 01ce6876-c420-4f11-8d20-d3ea61386d4e
• exact candidate: 0382feb62325c0f6c778342a77e31884ae3615de
• candidate tree: b9494aff721273c28c2127e296d3e481358db9e2
• reviewed parent: 73370af3f31dbe474c8587a6e2b866a19cce1839

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