Add optional displayName field to plugin manifest

[aaronpowell]

Summary

Adds an optional displayName field to the plugin manifest, giving plugins a
human-readable label that is separate from the constrained machine identifier
name (§5.5: lowercase a-z0-9-., 1–64 chars, no leading/trailing hyphens or
consecutive separators).

Closes #16

Changes

• Add displayName to the JSON schema (schemas/1.0.0/plugin.schema.json).
• Document displayName in the metadata field table (§5.4) and add a dedicated
  subsection (§5.7).
• Include displayName in the full example manifest (§5.5).
• Clarify §5.3 so name is described as the plugin identifier (see §5.5
  constraints) rather than "human-readable", now that displayName fills the
  human-readable role.

Behavior

• displayName is optional; if present it MUST be a string.
• Hosts MAY use displayName in user interfaces instead of name.
• If absent, hosts MUST fall back to name as the display name.

Example

{
  "$schema": "https://open-plugins.com/schemas/1.0.0/plugin.schema.json",
  "name": "plugin-name",
  "displayName": "Plugin Name",
  "version": "1.2.0"
}