NAP-02: Chat direct message open topic

[dskvr]

Summary

• Adds NAP-02 for the chat:open-dm IFC topic.
• Defines { pubkey, displayName? } as the direct-message navigation payload.

Why

• Direct-message navigation needs explicit topic semantics independent of generic IFC transport.

Spec / API surface

• Discovery: shell.supports("ifc", "NAP-02").
• Manifest dependency remains ["requires", "ifc"].

Compatibility

• Existing IFC implementations can continue using the same topic string and JSON payload.

Validation

• git diff --cached --check passed before commit.
• Checked for private implementation references in the spec and README row.

Non-goals

• Encryption, relay selection, chat history retrieval, shell window policy, and generic IFC transport.

Follow-ups

• Companion topic PRs cover profile and stream context semantics separately.

Supersedes the chat:open-dm portion of the closed umbrella topic payload PR #20.

[dskvr]

Closing in favor of the namespace-family shape requested after review. The current replacement packet keeps profile:* in #21 and stream:* in #26; chat:open-dm is deferred rather than kept as a separate tiny numbered NUB in this round.