docs(sse): document SSE directive for v1.20.0

[ErickXavier]

What changed

SSE (Server-Sent Events) directive documentation synced from Core implementation (NOJS-270 initiative, task NOJS-277).

Context

• Initiative: NOJS-270 (SSE directive, v1.20.0)
• Task: NOJS-277 -- Skill sync

Changes

SKILL.md

• Added SSE cheat-sheet section under HTTP / Data Fetching with attribute table covering all 9 attributes
• Included compact live-feed example demonstrating sse-insert, sse-limit, $sse state, and error template
• Added sse.md entry to the Reference File Map

references/directives/sse.md (new)

• Full attribute API with defaults, types, and examples
• $sse connection state object with state transition table
• Data parsing (JSON with raw-string fallback)
• Reactive URL behavior (close + reopen on change, array reset)
• Authentication limitations (no custom headers; query tokens or cookies via sse-credentials)
• Connection limit tracking and HTTP/1.1 6-connection warning
• Disposal and cleanup (Safety Rules 1, 2, disconnection guards, watcher cleanup)

All claims cross-referenced against the actual src/directives/sse.js implementation on origin/feat/NOJS-270.

Files modified

• nojs/SKILL.md
• nojs/references/directives/sse.md (new)