[OC-021] Define and implement IPC v1 framing

[tcballard]

Gate: G1 — Nostr-only product

Planning source: validated implementation plan PR #1

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• Depends on: OC-002, OC-010.
• Deliverable: versioned JSONL request/response/event types, bounded decoder, hello negotiation, correlation IDs, subscriptions, and mode-0600 socket server under systemd runtime directory.
• Acceptance: malformed/oversized lines cannot exhaust the daemon; slow subscribers are bounded/disconnected; reconnect and version mismatch are tested.
• Excludes: all command handlers beyond health/status fixtures.

Maintainer reconciliation — 2026-09-04

Next closure evidence/work: Retain bounded framing/subscriber evidence; resolve socket publication via #225. Reconcile v1 wording if #226 lands. Client event consumption remains separately tracked in #223.

Baseline reviewed: main 87c3bcb7d65e3e2ab8ae317694b751f8efe9a8ed. Implementation presence is not a live conformance claim.